A purchase order (PO) is a document a buyer issues to authorise a purchase, listing what is being bought, the quantities, the agreed prices and a PO number. The buyer creates it; the seller's job is to quote it. On the invoice, the PO number goes in the header next to the invoice number and date, or in a field labelled “PO number”, so the buyer's accounts team can match the invoice to the order and the delivery before approving payment. If a client works with POs, ask for the number before you start, not when you invoice. Example: the client sends PO 4500012345 for 12 hours of consulting at €90; invoice INV-0231 quotes “PO 4500012345” and bills 12 hours at €90, €1,080 excluding tax. Common mistakes: invoicing more than the PO covers, when extra work normally needs a new or amended PO first, and mistyping the PO number or leaving it off, which can leave the invoice unmatched in the client's system. A PO is not an invoice and doesn't replace one.
Read more: How to write an invoice (step by step).