High Quality Content by WIKIPEDIA articles! A purchase order (PO) is a commercial document issued by a buyer to a seller, indicating types, quantities, and agreed prices for products or services the seller will provide to the buyer. Sending a purchase order to a supplier constitutes a legal offer to buy products or services. Acceptance of a purchase order by a seller usually forms a one-off contract between the buyer and seller, so no contract exists until the purchase order is accepted.