How does a Purchase Order work?

A purchase order (PO) is a everyday business document, but it has real legal significance. Used well, it brings clarity and control to buying goods and services, and it can form a binding contract. Here is how the purchase order process works in England and Wales, and the legal points to watch.

What a purchase order is

A purchase order is a document a buyer sends to a supplier to order goods or services, setting out what is wanted, the quantity, the agreed price, delivery details and terms. It creates a clear written record of the order.

The legal status: offer and acceptance

In contract terms, a purchase order is usually an offer by the buyer to purchase on the stated terms. A binding contract is generally formed when the supplier accepts it, by confirming the order, or often by performing (delivering the goods or starting the work). At that point both sides are bound, so the detail on the PO matters. Note: this is the usual position, but not always. If the supplier's quotation was the offer, or if the parties already have a framework or master agreement, the purchase order may instead operate as an acceptance of that offer or as an order placed under an existing contract.

Watch the "battle of the forms"

A common trap is the battle of the forms: the buyer issues a PO on its terms, and the supplier responds with an acknowledgement on its terms. Whose terms govern? In a battle of the forms, the court decides objectively whether and when a contract was formed and on which terms. Often the last set of terms sent and accepted by the other party's words or conduct will govern, but that is not an automatic rule, especially where the parties already have a framework agreement or other agreed terms. To avoid uncertainty, make clear which terms apply, and don't assume your PO terms automatically win.

The typical purchase order process

1. Requisition, an internal request to buy something.
2. PO created, with a unique PO number, description, quantity, price, delivery and terms.
3. PO sent to the supplier (the offer).
4. Supplier accepts, by confirmation or by performing.
5. Goods/services delivered, checked against the PO.
6. Invoice received, matched to the PO and delivery (the "three-way match"), then paid.

PO vs invoice

People sometimes confuse the two:

• a purchase order is created by the buyer to order goods/services (before supply); while
• an invoice is created by the supplier to request payment (after supply).

Matching the PO, delivery note and invoice helps prevent overpayment and fraud.

Why use purchase orders?

• Clarity, everyone knows exactly what was ordered, at what price.
• Control, POs support budgeting and authorisation.
• Evidence, a clear record if a dispute arises about what was agreed.
• Efficiency, they streamline approvals and accounts.

Key takeaways

• A purchase order is usually an offer to buy; a contract forms when the supplier accepts (often by performing).
• The terms on the PO matter, beware the battle of the forms where the supplier responds on its own terms.
• A PO orders goods (buyer, before supply); an invoice requests payment (supplier, after supply).
• POs bring clarity, control, evidence and efficiency to procurement.

Sources

• English contract law on offer and acceptance, and the "battle of the forms"
• Standard procurement practice (purchase order vs invoice; three-way matching)

--- This article is general information about the law of England & Wales as at 2026, not legal advice. For advice on your circumstances, consult a qualified solicitor.