It is a document that is issued by the purchaser of goods and services, confirming that the order is indeed valid and has been fulfilled by the seller. As such, it should be signed by the buyer as well as purchased from the seller. The due date is usually the date by which the buyer is expected to pay for the goods. A purchase invoice is an official document issued by a buyer to a vendor after the purchase of the goods. An invoice has due dates for different stages of transaction such as payment, delivery, and cancellation. Most of these documents are issued in a different format from invoices issued.