See also Voucher: an invoice is within the European union primarily legally defined by the EU VAT directive as an accounting voucher (to verify tax and VAT reporting) and secondly as a Civil law (common law) document.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.