Purchase orders and invoices often get confused, especially when working with larger clients. Here's how they differ.
What a purchase order is
A purchase order (PO) is issued by the buyer before work begins, authorizing the purchase and specifying agreed terms, quantities, and pricing internally within their organization.
What an invoice is
An invoice is issued by the seller after work is delivered, requesting payment — often referencing the PO number to confirm it matches an approved order.
Why POs matter for freelancers
If a client issues POs, always get one before starting work, and include the PO number clearly on your invoice — many companies won't process payment without this match.
What happens without a PO
Some smaller clients don't use a formal PO system at all — in that case, a written agreement over email serves the same confirming purpose before you invoice.
The key relationship
A PO authorizes the purchase; an invoice requests payment for it. Together, they create a clear paper trail from agreement to payment.