A PO invoice is an invoice that is generated and submitted by a supplier after receiving a valid purchase order from a buyer. Once the supplier fulfills the order and delivers the products or services, they create a PO invoice to request payment from the buyer. A non-PO invoice, on the other hand, is an invoice that a supplier submits without requiring a prior purchase order. This is often used for ad-hoc services, small purchases, or situations where a formal purchase order process might be impractical. Non-PO invoices are typically processed based on a service receipt, delivery receipt, or other verification forms, and they require approval before payment is made.