26 U.S.C. § 4461: Imposition of tax

(a) General ruleThere is hereby imposed a tax on any port use. (b) Amount of taxThe amount of the tax imposed by subsection (a) on any port use shall be an amount equal to 0.125 percent of the value of the commercial cargo involved. (c) Liability and time of imposition of tax(1) LiabilityThe tax imposed by subsection (a) shall be paid by—(A) in the case of cargo entering the United States, the importer, or (B) in any other case, the shipper. (2) Time of impositionExcept as provided by regulations, the tax imposed by subsection (a) shall be imposed at the time of unloading.