What is a purchase order, and how is it different from an invoice?

A purchase order is issued by the buyer and specifies what is being ordered, quantities, agreed prices, delivery terms, and a PO number. Once accepted it functions as a binding commitment to purchase. An invoice flows the other direction, issued by the seller after delivery, requesting payment for what was provided. The sequence in a procurement-driven organization is purchase order, then delivery, then invoice referencing that purchase order, then payment.

For anyone selling to mid-sized or large organizations, this matters more than it seems. Accounts payable in those companies matches invoices against purchase orders and receiving records before paying, a process called three-way matching. An invoice arriving without a valid PO number frequently cannot be processed at all, and the vendor discovers this weeks later when payment has not arrived and nobody flagged it. The practical rule is to obtain the purchase order before beginning work, confirm the PO number appears on every invoice, and verify that the PO amount covers the full scope, since work exceeding the PO value often cannot be paid without a change order.

Related documents fill out the workflow. A quote or estimate precedes the PO and is not binding. A statement of work defines scope in detail for services and is often referenced by the PO. A packing slip or delivery receipt evidences receipt. A statement summarizes outstanding invoices and is not itself a request for payment on a specific item. For small businesses selling to individuals or other small businesses, purchase orders are usually unnecessary overhead and a signed proposal or contract serves the same function. For anyone whose growth plan involves enterprise clients, learning the procurement process is a practical prerequisite, because the reason large-company invoices go unpaid is almost never unwillingness and almost always a process step nobody explained.