Glossary
What is a purchase order and why do large clients require one for invoicing?
A purchase order is a formal document issued by a buyer authorizing a specific purchase at an agreed price; it serves as the buyer's internal control and is often required by corporate accounts payable departments before an invoice can be processed.
A PO is initiated by the buyer, not the seller. When a department at a large company approves a purchase, they submit a requisition internally, procurement issues a PO with a unique number, and that PO number becomes the reference that ties the vendor's invoice to the approved spend. Accounts payable systems are often configured to refuse invoices that lack a matching PO number — not as a delay tactic, but as a fraud-prevention and budget-control mechanism.
For vendors, the PO number serves as powerful evidence that the purchase was authorized. If a client later claims they never approved the work, a PO with their internal tracking number proves otherwise. It also typically specifies the agreed scope, quantity, and price — reducing the surface area for disputed invoices.
Service providers who work with corporate clients should build PO collection into the project kickoff process, not the invoicing step. Ask for the PO number when the project is approved, not when you're ready to bill. A missing PO at invoice time means your payment clock hasn't started — the client won't process payment until they generate one, and generating it can take days or weeks inside a large organization.
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