Where Customs Duties are, at the Union, leviable on any Goods, Wares, or Merchandises in any Two Provinces, those Goods, Wares, and Merchandises may, from and after the Union, be imported from one of those Provinces into the other of them on Proof of Payment of the Customs Duty leviable thereon in the Province of Exportation, and on Payment of such further Amount (if any) of Customs Duty as is leviable thereon in the Province of Importation.