Can you invoice without a PO?

Yes — invoicing without a PO is perfectly legal and extremely common, especially in small business. A contract can exist through email agreement, a signed quote or plain verbal agreement; the invoice just bills for it. Utilities, subscriptions, professional services and most B2C transactions never involve a PO at all.

The catch is on the buyer's side: a 'non-PO invoice' has nothing to be checked against, so approving it relies on someone remembering what was agreed. That's where overbilling and duplicate invoices slip through. Larger companies often enforce 'no PO, no pay' policies precisely for this reason — if you supply bigger customers, expect to need their PO number on every invoice.

Sensible practice: allow non-PO invoices for small operating expenses, require POs for inventory and anything negotiated.