An invoice , bill or tab is a commercial document issued by a seller to a buyer , relating to a sale transaction and indicating the products , quantities, and agreed prices for products or services the seller had provided the buyer. Payment terms are usually stated on the invoice. These may specify that the buyer has a maximum number of days in which to pay and is sometimes offered a discount if paid before the due date. The buyer could have already paid for the products or services listed on the invoice. From the point of view of a seller, an invoice is a sales invoice. From the point of view of a buyer, an invoice is a purchase invoice.