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What is a purchase order?
A purchase order (PO) is a buyer's formal document authorizing a purchase — what they want, how many, at what price. The supplier fulfills it and invoices against the PO number, so the buyer's accounts payable can match all three. No PO, no payment is a common rule at larger companies.
In practice
A school district sends PO-7741 for 30 laptops at $620 each ($18,600). The vendor ships, then invoices 'per PO-7741' — the district's AP clerk matches PO, delivery note, and invoice before releasing payment.
Invoice against any PO
Reference the PO number on a clean, professional invoice and get paid without AP delays.
Create an invoice — freeFrequently asked questions
Do I need a purchase order to invoice a company?
Ask before starting work. Many mid-size and large companies won't pay without one — find out during onboarding, not at invoice time.
Where does the PO number go on an invoice?
Near the top, labeled 'PO number' or 'Reference.' It's the first thing the client's AP department looks for.
What's the difference between a PO and a contract?
A contract governs the relationship; a PO authorizes a specific purchase under it. Big engagements usually have both.