Article 1, Section 10, Clause 2: Import-Export Clause

No State shall, without the Consent of the Congress, lay any Imposts or Duties on Imports or Exports, except what may be absolutely necessary for executing it’s inspection Laws: and the net Produce of all Duties and Imposts, laid by any State on Imports or Exports, shall be for the Use of the Treasury of the United States; and all such Laws shall be subject to the Revision and Controul of the Congress.

This clause reserves all revenue arising from duties or imposts on foreign trade to the federal government. It does not prohibit states from imposing taxes on particular classes of goods prior to their entering the export stream or on imported goods after they have become “mingled” with the internal economy of a state.  In the case of imported goods, the courts have placed the line at the point where the goods are broken out of their packaging by the purchaser, which is probably as reasonable a place to put the line as any.