Imported goods are generally accompanied by a bill of lading or air waybill describing the goods. For purposes of customs duty assessment, they must also be accompanied by an invoice documenting the transaction value. The goods on the bill of lading and invoice are classified and duty is computed by the importer or CBP. The amount of this duty is payable immediately, and must be paid before the goods can be imported. Most assessments of goods are now done by the importer and documentation filed with CBP electronically.