If a supplier delivers goods and then suddenly sends a higher invoice, adds a “price adjustment,” or refuses to honor the agreed price, the most important thing is not to panic or immediately pay the new amount. In the Philippines, the answer usually depends on one question: was there already a clear agreement on the item and the price before delivery? If yes, the supplier generally cannot change the price after delivery unless your contract allows it or you later agree to the change. This article explains how Philippine law treats post-delivery price changes, what documents matter, how to respond in writing, when to pay or withhold payment, and where to bring the dispute if it cannot be settled.

The short answer: a supplier usually cannot change pricing after delivery

Under Philippine contract law, a deal is not just a casual promise once the essential terms are agreed. The Civil Code provides that obligations arising from contracts have the force of law between the parties and must be complied with in good faith. (Lawphil)

For a sale of goods, the Civil Code is even more specific: a contract of sale is perfected when there is a meeting of minds on the thing sold and the price. From that moment, the parties may demand performance from each other. (Lawphil)

So if your purchase order, quotation acceptance, supply agreement, chat confirmation, email thread, or signed contract clearly shows:

the goods or materials ordered;
the agreed unit price or total price;
the quantity;
delivery terms; and
payment terms,

then a supplier’s later invoice at a higher price is not automatically binding.

A supplier may have business reasons for wanting a higher price: fuel costs, foreign exchange changes, raw material increases, customs charges, or a mistake in the quotation. But a business reason is not the same as a legal right to unilaterally change the price.

When a post-delivery price increase may be valid

There are situations where a price change after delivery can be valid. The key is whether the buyer agreed to that possibility before or after delivery.

1. Your contract has a price adjustment clause

Some supply contracts allow price changes based on a formula, index, exchange rate, fuel surcharge, commodity price, or agreed escalation mechanism.

Examples:

“Price is subject to adjustment based on actual landed cost.”
“Final price shall depend on the prevailing supplier price list on delivery date.”
“Quoted price valid only until June 30, 2026.”
“Buyer shall shoulder additional customs duties, freight increases, and government charges.”
“For repeat orders, seller’s current price at time of dispatch applies.”

If the clause is clear and was part of the agreement, the supplier may have a stronger argument.

But vague language is different. A small note saying “price may change without notice” may not automatically defeat a specific accepted purchase order, especially if the final order already fixed the price and the supplier delivered without objection.

2. You agreed to the new price after delivery

A buyer can agree to a new price after delivery. This may happen by:

signing a revised quotation or invoice marked “conforme”;
sending an email saying “approved” or “we accept the revised price”;
paying the higher amount without protest;
issuing a new purchase order at the higher price;
negotiating and signing a settlement; or
continuing a course of dealing where both sides consistently treated delivery-date pricing as final.

This is why buyers should be careful with delivery receipts, invoices, and “received” forms. If a document contains a new price or new payment terms, do not sign it blindly.

A safer notation is:

Received goods only. Price adjustment not accepted. Subject to reconciliation against PO/contract.

3. There was no final agreed price

Sometimes the parties agreed on the goods but not on the final price. This happens in informal transactions, rush orders, long-term supplier relationships, or verbal arrangements such as:

“Deliver muna, invoice na lang after.”
“Same as last time unless may changes.”
“Use market price on delivery date.”
“Final costing to follow.”

In these cases, the supplier may not be completely barred from billing a different price. Under the Civil Code, if the price cannot be determined but the goods have been delivered and appropriated by the buyer, the buyer must pay a reasonable price, and what is reasonable depends on the facts. (Lawphil)

A “reasonable price” is not whatever the supplier later chooses. It may be proven by:

previous invoices between the parties;
market prices at the time of delivery;
price lists;
supplier quotations from comparable sellers;
landed cost documents;
trade usage;
the parties’ past practice; and
the buyer’s conduct after receiving the goods.

4. There was a genuine change order or additional work

If the buyer requested extra goods, faster delivery, special packaging, substitution of materials, installation, warehousing, after-hours delivery, or other additions not covered by the original order, the supplier may charge more if those extras were agreed.

The dispute often becomes factual: was the additional charge truly for extra work, or just a disguised price increase?

5. Government procurement rules allow limited price escalation

If the buyer is a government agency, local government unit, GOCC, SUC, or other covered procuring entity, stricter procurement rules apply. Under Republic Act No. 12009, the New Government Procurement Act, bid prices for the awarded scope of work are generally fixed and not subject to escalation during contract implementation. For infrastructure projects, price escalation may be considered only for extraordinary increases, subject to the conditions stated in the law and prior approval of the Government Procurement Policy Board. (Lawphil)

Older contracts governed by Republic Act No. 9184 followed a similar principle: bid prices were generally fixed, with price escalation allowed only under extraordinary circumstances and prior GPPB approval. (Supreme Court E-Library)

For government contracts, the supplier cannot simply send a higher invoice after delivery and expect payment outside the procurement rules, contract, inspection and acceptance process, and applicable COA rules.

What Philippine law says about agreed prices and payment

Key Civil Code rules that usually apply

Several Civil Code provisions are especially relevant when a supplier changes pricing after delivery.

Legal rule	Practical meaning
Article 1159: Contracts have the force of law between the parties and must be complied with in good faith.	If the agreed price is ₱500,000, neither side can casually rewrite it after delivery. (Lawphil)
Article 1305: A contract is a meeting of minds where one party binds himself to give something or render service.	The real issue is what both parties actually agreed to, not what one side later wanted. (Lawphil)
Article 1306: Parties may set terms as they wish, as long as they are not contrary to law, morals, good customs, public order, or public policy.	Price escalation clauses, foreign exchange clauses, and delivery-date pricing can be valid if properly agreed. (Lawphil)
Article 1308: The validity or compliance of a contract cannot be left to the will of one party.	A supplier generally cannot reserve unlimited power to decide the final price alone. (Lawphil)
Article 1475: A sale is perfected once there is agreement on the thing and the price.	Once goods and price are agreed, both sides can demand performance. (Lawphil)
Article 1473: Fixing the price can never be left to the discretion of one contracting party, unless the other accepts it.	“We changed the price because we said so” is weak unless the buyer accepted the new price. (Lawphil)
Article 1474: If price cannot be determined but goods were delivered and appropriated, the buyer must pay a reasonable price.	If there was no fixed price, the buyer may still owe a fair amount. (Lawphil)
Article 1582: The buyer must accept delivery and pay the price at the agreed time and place. If not stipulated, payment is made at delivery.	A buyer who accepts conforming goods should be ready to pay the agreed price, even while disputing the added amount. (Lawphil)
Article 1169: Delay usually begins after judicial or extrajudicial demand.	A proper written demand can start default consequences. (Lawphil)
Article 1170: Fraud, negligence, delay, or contravention of the obligation can give rise to damages.	A party that breaches the agreed price or refuses proper payment may face damages. (Lawphil)

Step-by-step: what to do if your supplier changes the price after delivery

1. Gather every document before replying

Do not rely on memory. In pricing disputes, documents usually decide the issue.

Collect:

signed supply agreement;
quotation and revised quotations;
purchase order;
order confirmation;
delivery receipt;
sales invoice or billing statement;
official receipt or proof of partial payment;
email and Viber/WhatsApp/Messenger messages;
screenshots of online listings or platform prices;
price lists;
proof of delivery date and quantity;
inspection and acceptance reports;
photos of goods received;
prior invoices for the same items;
proof of market price from other suppliers; and
internal approvals or board/management authority, if a company is involved.

For businesses, retrieve the original PO approval trail. Many disputes arise because the procurement staff approved one price, while accounting later receives a different invoice.

2. Identify the exact pricing issue

Be precise. “Supplier overcharged us” is too general.

Classify the problem:

Situation	Main legal question
Supplier delivered goods, then billed a higher unit price	Was there already an agreed unit price?
Supplier added freight, VAT, customs, or handling fees	Were these included or excluded in the agreed price?
Supplier claims raw material costs increased	Was there a price escalation clause?
Supplier says the original quotation expired	Was the quotation accepted before expiration?
Supplier says the quote was a mistake	Was the mistake obvious, material, and raised promptly?
Supplier delivered a substitute item at a higher price	Did the buyer approve the substitution and price?
Buyer accepted extra quantity	Was the extra quantity ordered or knowingly accepted?
Supplier refuses warranty unless higher price is paid	Is the supplier withholding a separate obligation to pressure payment?

This classification helps you avoid emotional responses and focus on proof.

3. Check whether the delivery receipt changed the contract

In the Philippines, many delivery receipts are signed quickly by guards, warehouse staff, site engineers, or household helpers. The signature usually proves receipt of goods, not necessarily acceptance of new commercial terms.

Still, read the document carefully. Some delivery receipts or invoices contain language such as:

“Received in good order and condition.”
“Customer accepts price stated herein.”
“Subject to seller’s standard terms.”
“All sales final.”
“Interest applies after due date.”
“No claims after 24 hours.”

If your staff signed a document with a new price, the supplier may argue acceptance. Your response should explain the authority issue clearly: the receiving staff may have authority to acknowledge delivery, but not to approve a new price.

For companies, this is why supplier contracts should state who may approve price changes.

4. Do not ignore the invoice

Silence can create problems. If the supplier sends a higher invoice and you do nothing, the supplier may later argue that you accepted it, especially if you used the goods, sold them onward, or continued ordering.

Reply in writing as soon as possible. A practical response should:

acknowledge receipt of the invoice;
identify the original agreed price;
state that you dispute only the increased portion;
attach the PO, quotation, or contract;
request a corrected invoice;
offer to pay the undisputed amount, if appropriate;
reserve your rights; and
avoid insulting or threatening language.

Example wording:

We acknowledge receipt of your Invoice No. ___ dated . We note that the billed unit price of ₱ differs from the agreed unit price of ₱___ under PO No. ___ / quotation dated . We do not accept the post-delivery price adjustment. Please issue a corrected invoice based on the agreed price. Without prejudice to our rights and defenses, we are prepared to settle the undisputed amount of ₱ upon receipt of the corrected billing.

5. Consider paying the undisputed amount

If you received conforming goods and there is no dispute that the original price is due, paying the undisputed amount can be a smart move.

It shows good faith and reduces the risk that the supplier will claim you are the one in default. Under the Civil Code, a buyer is bound to pay the price at the agreed time and place, or at delivery if no time and place were stipulated. (Lawphil)

When paying the undisputed amount, use clear payment language:

“Payment of undisputed contract price only.”
“Without prejudice to dispute on price adjustment.”
“Not an admission of revised invoice.”
“Subject to reconciliation.”

Keep proof of payment and request the appropriate invoice or receipt.

6. If you must withhold payment, document the legal reason

Withholding the entire payment can be risky if the goods were delivered and accepted. The supplier may charge interest, suspend supply, or file a collection case.

Withholding may be more defensible if:

the supplier delivered the wrong goods;
there is a shortage in quantity;
the goods are defective;
the supplier refuses to correct the invoice;
the price was never agreed and the billed amount is unreasonable;
the supplier is demanding an unlawful or unauthorized charge;
the contract allows set-off or retention;
the supplier failed inspection or acceptance requirements; or
payment depends on documents the supplier has not submitted.

If the issue is defective goods, short delivery, or nonconforming goods, document it immediately through photos, inspection reports, written notice, and segregation of the goods if possible.

7. Send a formal demand or dispute letter

If informal communication fails, send a formal letter. It does not always need to be notarized, but notarization can help prove identity and date, especially for serious disputes.

A good letter should include:

Names and addresses of the parties.
Contract, PO, quotation, invoice, and delivery reference numbers.
Chronology of events.
Agreed price and disputed revised price.
Legal and factual basis for rejecting the increase.
Demand for corrected invoice, refund, delivery completion, or cessation of collection.
Deadline to respond, often 5 to 10 working days in commercial practice.
Reservation of rights.
Attachments.

Send it by a trackable method:

personal service with receiving copy;
registered mail;
courier;
email with delivery/read receipt;
platform complaint system; or
all of the above, depending on urgency.

A written demand can also matter for default and interest issues because delay under Article 1169 is generally tied to judicial or extrajudicial demand. (Lawphil)

8. Preserve your business position

Pricing disputes can disrupt operations. While asserting your rights, think practically:

Do you need this supplier for ongoing operations?
Is there an alternative supplier?
Are the goods perishable?
Are you reselling the goods to customers?
Will delay cause penalties under your own downstream contracts?
Is the disputed amount worth litigation?
Is a commercial compromise cheaper than a lawsuit?

Many Philippine supplier disputes are resolved through credit memos, offsetting in the next order, installment adjustments, or settlement agreements. If you settle, put the agreement in writing and specify that it is a full settlement of the disputed invoice.

Where to bring the dispute in the Philippines

The proper venue depends on who the parties are, the amount involved, and whether the transaction is consumer, commercial, online, or government-related.

Forum	When it may apply	Practical notes
Direct negotiation	Most supplier pricing disputes	Fastest and cheapest. Always document the result.
Barangay conciliation	Covered disputes between individuals actually residing in the same city or municipality	Not for corporations, partnerships, juridical entities, government-related disputes, and other excluded cases. Prior barangay conciliation can be a pre-condition to court action in covered cases. (Lawphil)
DTI consumer complaint	Natural-person consumers dealing with consumer products or services	Consumer complaints must generally be filed within two years from the transaction or from discovery of hidden defects; Consumer Arbitration Officers handle mediation, conciliation, hearing, and adjudication of covered complaints. (Supreme Court E-Library)
DTI online/e-commerce channels	Online seller, online merchant, e-retailer, or platform-related transaction	DTI states that complaints against online sellers may be sent to the Fair Trade Enforcement Bureau, and that FTEB accommodates complaints for online and offline businesses. (DTI ECommerce)
Small Claims Court	Money claim not exceeding ₱1,000,000	The Supreme Court increased the small claims threshold to ₱1,000,000. Claims may include money owed under services and sale of personal property. (Supreme Court of the Philippines)
Regular civil action / summary procedure	Larger or more complex claims	First-level courts now handle many civil monetary claims up to ₱2,000,000 under expanded jurisdiction and expedited procedures. (Lawphil)
Arbitration	Contract has an arbitration clause, or parties agree to arbitrate	Common in construction, distribution, and larger commercial contracts.
Government procurement remedies	Buyer is a government procuring entity	Follow the contract, procurement law, GPPB rules, dispute resolution clause, inspection/acceptance process, and COA requirements.

DTI complaints: when the buyer is an ordinary consumer

If the buyer is an ordinary person buying for personal, family, or household use, a sudden price change after delivery may fall within consumer protection concerns, especially if the seller misrepresented the price, failed to disclose charges, or used misleading online listings.

Republic Act No. 7394, the Consumer Act of the Philippines, remains a key law for consumer protection. The DTI’s own consumer materials identify the consumer’s right to information and right to redress, including protection against misleading advertising, labeling, and promotion. (DTI ECommerce)

For online transactions, Republic Act No. 11967, the Internet Transactions Act of 2023, covers business-to-business and business-to-consumer internet transactions within DTI’s mandate. It also provides remedies for online consumers in cases involving defects, warranty issues, or liabilities arising from the contract, including repair, replacement, refund, or other remedies under the Consumer Act and other laws. (Supreme Court E-Library)

For DTI complaints, prepare:

government-issued ID;
proof of purchase;
screenshots of listed price;
order confirmation;
invoice or billing statement;
delivery proof;
messages with the seller;
proof of payment;
photos of goods, if relevant;
written demand or complaint narrative; and
name, address, platform page, phone number, and email of the seller.

If the seller is abroad but actively sells to Philippine customers through a platform, preserve platform records immediately. Online listings can disappear.

Small claims: when the dispute is mainly about money

If the dispute is a straightforward money claim, small claims may be the practical route.

The Supreme Court’s small claims page provides downloadable forms, including the Statement of Claim, Response, notice, summons, motion for execution, and related forms. (Supreme Court of the Philippines)

Small claims may be useful when:

the buyer paid the higher amount and wants a refund;
the supplier sues for the disputed increase;
the buyer owes only the original price but the supplier claims more;
the dispute is based on invoices, POs, delivery receipts, and written communications; and
the amount does not exceed ₱1,000,000.

Typical documents include:

Document	Why it matters
Statement of Claim or Response	Main small claims form filed with the court.
Contract, PO, quotation, or order confirmation	Shows the agreed price and terms.
Delivery receipt	Shows what was delivered and when.
Invoice and revised invoice	Shows the price change.
Proof of payment	Shows what was paid or tendered.
Demand letter and reply	Shows the dispute and attempts to settle.
Screenshots and emails	Shows representations, acceptance, and timing.
Barangay Certificate to File Action, if required	Avoids dismissal or delay for covered disputes.
Secretary’s Certificate or Board Resolution, if a corporation files	Shows authority of the representative.
Special Power of Attorney, if represented	Often needed if a representative appears.

For corporate buyers and suppliers, check the exact authority documents required by the court. A company representative should not appear without proof that the corporation authorized the filing, settlement, or defense.

Common scenarios and how to analyze them

Scenario 1: The supplier says, “Our cost increased before delivery”

Ask:

Was the price fixed in the PO or contract?
Was there a price escalation clause?
Did the supplier notify you before delivery?
Did you approve the increase before accepting delivery?
Did the supplier have a right to cancel instead of delivering?

If the supplier delivered after accepting a fixed-price PO, a later cost increase usually does not automatically change the buyer’s price.

Scenario 2: The supplier says, “The quotation expired”

Check the quotation validity date and acceptance date.

If the buyer accepted after expiration, the supplier can argue there was no binding acceptance at the old price. If the supplier nevertheless confirmed the order, delivered the goods, or acted as if the order was accepted, the buyer may argue the supplier waived the expiration or accepted the order through conduct.

Scenario 3: The supplier delivered first, then issued the only invoice

If there was no prior quotation, PO, or price agreement, the issue may be reasonable price. Compare the invoice with market prices, previous dealings, and industry practice.

Do not simply refuse to pay. A buyer who used the goods may still owe a reasonable amount.

Scenario 4: The invoice includes surprise freight, VAT, or handling charges

Check whether the quoted price was:

VAT-inclusive or VAT-exclusive;
delivered price or ex-warehouse price;
inclusive of freight, unloading, and handling;
subject to customs duties;
subject to installation or commissioning fees; or
governed by Incoterms or similar shipping terms.

If the supplier’s quotation was unclear, the parties’ prior practice becomes important. For example, if all previous invoices included delivery in the unit price, that helps the buyer.

Scenario 5: The buyer signed “conforme” on the revised invoice

This is dangerous for the buyer. “Conforme” usually means agreement.

The buyer may still argue mistake, lack of authority, fraud, or that the signature was for receipt only, but the evidentiary burden becomes harder. The Civil Code recognizes fraud where one party is induced by insidious words or machinations to enter into a contract he would not have agreed to. (Lawphil)

Scenario 6: The supplier threatens a criminal case if the buyer does not pay the higher price

Most supplier pricing disputes are civil, not criminal. A buyer who disputes a price in good faith is not automatically committing estafa.

However, criminal risk may arise if there was fraud from the beginning, such as ordering goods with no intent to pay, using false identity, issuing fraudulent payment proof, or deceiving the supplier into releasing goods. Article 315 of the Revised Penal Code covers estafa or swindling in specific situations involving deceit, abuse of confidence, or fraudulent means. (Lawphil)

The practical point: respond calmly, document the dispute, and pay or tender the undisputed amount when appropriate.

Scenario 7: The supplier is foreign or the buyer is abroad

For cross-border transactions, check:

governing law clause;
venue or jurisdiction clause;
arbitration clause;
Incoterms or shipping terms;
currency and exchange rate clause;
customs and duties allocation;
platform dispute rules;
service of notices; and
authentication of foreign documents.

If documents executed abroad must be used in the Philippines, notarized foreign documents may need an apostille or consular authentication depending on the country. DFA guidance explains the Philippine Apostille process and documentary requirements for public documents. (Apostille Philippines)

Practical mistakes to avoid

Do not sign revised invoices without reading them

A signature can be used as evidence of acceptance. Train warehouse, site, and receiving staff to sign only for receipt, not price approval.

Do not rely only on phone calls

After every call, send a short written recap:

As discussed today, we maintain that PO No. ___ fixed the price at ₱___. We do not accept the revised invoice. Please confirm issuance of corrected billing.

Do not use all the goods while refusing to pay anything

If the goods are conforming and the original price is undisputed, using everything while paying nothing can weaken your position.

Do not mix separate accounts unless set-off is allowed

If you owe the supplier on another invoice, do not assume you can automatically offset unless the contract, law, or supplier agrees. Improper set-off can trigger a separate collection dispute.

Do not post accusations online without proof

Calling the supplier a scammer, fraudster, or criminal online can create defamation or business reputation issues. Keep complaints factual and send them to the proper forum.

Do not miss prescription or filing periods

Consumer complaints under the implementing rules of the Consumer Act must generally be filed within two years from the consumer transaction or from discovery of hidden defects. (Supreme Court E-Library)

Civil collection and contract claims have their own prescription rules depending on whether the obligation is written, oral, or based on other sources. Do not wait until documents, witnesses, and online records disappear.

How to prevent this in future supplier transactions

A simple purchasing system prevents many pricing disputes.

Use these safeguards:

Require written quotations with validity dates.
State whether price is VAT-inclusive or VAT-exclusive.
Specify whether delivery, freight, unloading, installation, and customs charges are included.
Use formal purchase orders with item description, quantity, unit price, total price, delivery date, and payment terms.
State that price changes require written approval from authorized officers.
Limit who can sign “conforme” documents.
Train receiving staff to write “received only” when signing delivery receipts.
Require suppliers to disclose price escalation clauses before acceptance.
Keep all supplier communications in email or documented messaging channels.
Reject or clarify revised invoices immediately.

A useful clause for buyers is:

Any change in price, quantity, specifications, delivery charges, taxes, fees, or payment terms shall be valid only if approved in writing by Buyer’s authorized representative before delivery. Acceptance of delivery shall not constitute acceptance of any price increase or new term stated in Seller’s invoice, delivery receipt, or other post-order document.

Frequently Asked Questions

Can a supplier legally increase the price after delivery in the Philippines?

Usually, no, if there was already a clear agreed price before delivery. A supplier may only have a valid basis if the contract allows price adjustment, the buyer agreed to the change, there was no final agreed price, or the increase relates to approved additional goods or services.

What if I accepted the delivery but not the new price?

Accepting delivery does not always mean accepting a new price. If you signed only to acknowledge receipt, say so in writing immediately. If the document you signed says “conforme” to the new invoice or terms, the supplier has a stronger argument that you accepted the revised price.

Should I pay the original price while disputing the increase?

Often, yes. If the goods were properly delivered and the original contract price is due, paying the undisputed amount shows good faith. Mark the payment clearly as payment of the undisputed contract price only and not acceptance of the revised invoice.

What if the supplier refuses to issue a corrected invoice?

Send a written dispute letter identifying the agreed price and attaching the PO, quotation, or contract. Ask for a corrected invoice. If needed, tender the undisputed amount and keep proof. If the refusal continues, consider DTI, barangay conciliation if applicable, small claims, arbitration, or civil action.

Can I return the goods instead of paying the higher price?

It depends. If the supplier breached the agreed price and you promptly reject the unauthorized increase, return may be possible by agreement or under applicable remedies. But if you used, consumed, resold, or benefited from the goods, you may still owe at least the agreed price or a reasonable price if no price was fixed.

What if the supplier says the old quotation was a mistake?

A minor internal mistake by the supplier does not automatically change the contract. But if the price was obviously erroneous, immediately corrected, or never validly accepted, the supplier may have arguments based on mistake or lack of meeting of minds. The facts and documents will matter.

Can the supplier charge interest on the higher amount?

The supplier can only charge contractual interest if there is a valid written basis. Legal interest may apply in some money claims after default or demand, but the supplier must first establish what amount is actually due. The Supreme Court has recognized the 6% per annum legal interest framework in the absence of stipulation, generally computed from default or as otherwise determined by the court. (Lawphil)

Is this a DTI complaint or a court case?

If you are an ordinary consumer buying consumer goods or services, DTI may be appropriate. If it is a business-to-business collection or invoice dispute, it may be a civil, small claims, arbitration, or commercial contract matter. If the buyer is a government entity, procurement rules and the contract’s dispute process must be checked.

Do foreigners have the same rights in Philippine supplier disputes?

Foreigners and foreign companies can generally enforce contracts in the Philippines, subject to procedural rules, jurisdiction, capacity, authentication of foreign documents, and any contract clauses on governing law or arbitration. If documents are executed abroad, apostille or consular authentication may be needed depending on the country and document type.

Can I sue in small claims for a supplier overcharge?

Yes, if the dispute is a money claim within the small claims threshold and the case fits the rules. The current small claims threshold is ₱1,000,000, and the Supreme Court provides official small claims forms. (Supreme Court of the Philippines)

Key Takeaways

A supplier generally cannot change the price after delivery if the goods and price were already agreed.
The strongest evidence is usually the purchase order, accepted quotation, contract, delivery receipt, invoice, and written messages.
Do not sign revised invoices, “conforme” forms, or delivery documents containing new prices unless you actually agree.
If the original price is undisputed, consider paying that amount “without prejudice” while rejecting the added charge in writing.
If no final price was agreed and you used the goods, Philippine law may require payment of a reasonable price.
Consumer buyers may consider DTI remedies; business disputes may go to negotiation, barangay if applicable, small claims, arbitration, or regular court.
Government procurement contracts have stricter rules: bid prices are generally fixed, and price escalation is limited by procurement law and GPPB rules.
The best protection is a written purchasing process that says price changes require prior written approval from an authorized person.