19 U.S.C. § 240

When the duty upon any imports shall be subject to be levied upon the true market value of such imports in the principal markets of the country from whence the importation has been made, or at the port of exportation, the duty shall be estimated and collected upon the value on the day of actual shipment, whenever a bill of lading shall be presented showing the date of shipment, and which shall be certified by a certificate of the United States consul or legally authorized deputy.