A PO is almost always an offer, not a contract. Whether it becomes one, and on whose terms, depends entirely on what happened next.
Your buyer sends a purchase order for 4,000 units at £12.40 each, on your company's standard terms printed on the reverse. The supplier emails back an order acknowledgement confirming the quantity and price, attaching their own standard terms, which cap liability at the invoice value and exclude all consequential loss. Nobody reads either document. The goods arrive, you pay, everyone is happy.
Eight months later a defective batch halts a production line for four days. Your terms say the supplier is liable for losses arising from defects. Their terms say liability is capped at £49,600. The loss is £380,000. Both parties are now looking at the same transaction and reading completely different contracts.
So: is a purchase order a contract? The lawyerly answer is that it depends, which is unhelpful. The useful answer is that a purchase order is almost always an offer, and whether it becomes a contract, and on whose terms, depends entirely on what happened next.
A purchase order is a document issued by a buyer to a supplier specifying what is being ordered: items, quantities, prices, delivery dates, delivery location, and usually a set of standard terms and conditions.
Legally, it is a commercial offer. It proposes definite terms and signals that the buyer intends to be bound if the supplier accepts. That distinguishes it from a request for quotation, which is an invitation for the supplier to make an offer, and from an internal requisition, which is just an authorisation to spend.
An offer, on its own, is not a contract. It is one half of one. For the full picture of what has to be present, the six essential elements of a valid contract is the underlying framework here — and offer and acceptance are the two doing the work.
A PO becomes a contract at the point of acceptance. Acceptance can happen in several ways, and they carry different degrees of certainty.
This is the heart of the problem, and it happens in the overwhelming majority of B2B transactions.
Under the mirror image rule, acceptance must match the offer exactly. Respond on different terms and you have made a counter-offer. So the standard exchange plays out like this: buyer sends PO on buyer's terms (offer). Supplier sends acknowledgement on supplier's terms (counter-offer, killing the original). Buyer says nothing but accepts delivery (acceptance by conduct of the counter-offer). Supplier's terms govern.
That outcome is known as the last shot doctrine, and it is the traditional English law position. Whoever fired the final set of terms before performance wins. It produces the counterintuitive result that the party who reads least carefully and responds last is often the party whose terms apply.
The approach is not universal:
The practical consequence for anyone trading internationally is that the same exchange of documents can produce different governing terms depending on where the dispute lands.
These three documents get conflated constantly, and they do different jobs.
The three-way match used in accounts payable — PO, goods receipt, invoice — exists precisely because these are distinct records of distinct events, and reconciling them catches errors.
A PO that is definite enough to accept, and specific enough to enforce, should carry:
A purchase order raised after the goods have already arrived does none of this work. The commitment was made, the terms were settled by conduct, and the PO is now a bookkeeping entry dressed as a control. High volumes of retrospective POs are a reliable signal that the procurement process is being routed around, usually because the compliant path is slower than the need.
A blanket purchase order covers repeated deliveries over a period, with releases called off against it. Whether it is binding depends on its content.
If it commits you to a minimum quantity or value, it is generally a contract for that commitment. If it merely sets prices and terms for orders you may or may not place, it is usually a framework rather than a binding purchase — there is no consideration for a promise to buy nothing. Suppliers sometimes assume a blanket order guarantees volume when it does not, and buyers sometimes assume it locks pricing when the supplier reserved a right to vary. Both assumptions are worth checking against the actual wording.
Not by itself. A purchase order is an offer, and an offer becomes binding only when accepted. Acceptance can be express (a signed acknowledgement) or by conduct (the supplier ships the goods or starts the work). Until one of those happens, the buyer can generally withdraw the order, provided the withdrawal is communicated before acceptance. Where a master agreement already governs the relationship, the PO is a call-off under that contract rather than a fresh offer, and it is binding from issue on the master agreement's terms.
A purchase order is a document that proposes a transaction. A contract is the legal relationship created when that proposal is accepted. A PO can become a contract, form part of one, or be a call-off under an existing one. Contracts also exist in plenty of situations where no purchase order was ever raised. The practical distinction is that a PO is transaction-specific and typically covers what, how many, when, and how much, while a negotiated contract covers the harder questions of liability, warranties, termination, and what happens when things go wrong.
It depends on the jurisdiction and the sequence. Under the traditional English law last shot approach, the final set of terms transmitted before performance generally governs — so if the supplier's acknowledgement was the last document sent before delivery, their terms are likely to apply. US law under the Uniform Commercial Code and many civil law systems take a knock-out approach instead, cancelling conflicting terms and substituting default rules. Because the outcome varies, the reliable fix is a master agreement that expressly overrides both parties' standard forms.
Before acceptance, generally yes — an offer can be revoked at any time before it is accepted, as long as the revocation reaches the supplier first. After acceptance, a contract exists and cancellation depends on its terms. Many purchase orders and supply agreements include cancellation provisions with charges reflecting work already done or materials committed. Cancelling without a contractual right is a breach, exposing you to the supplier's losses.
No. An invoice is a demand for payment issued after performance. It records an obligation that already exists rather than creating one. Terms printed on an invoice for the first time, after the goods were delivered and the bargain was struck, generally arrive too late to form part of the contract. This does not stop suppliers including them, and it does not stop buyers assuming they apply, but as a matter of formation the contract was already complete.
Not legally, but operationally it is usually worth it. The PO provides the commitment record, the budget check, the approval trail, and the reference point for receipting and invoice matching. Under a master agreement the PO is not doing contractual work — the master terms already govern — but it is doing financial control work. The important detail is that the PO should reference the master agreement explicitly, so nobody later argues the PO's own boilerplate displaced the terms you negotiated.
HERO keeps the agreement behind your purchase orders structured and queryable — so which master terms govern a given call-off, what the liability cap actually is, and whether an amendment changed it are lookups rather than a search through PDFs and email threads. Book a demo.