Article I, Section 10, Clause 2:
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.
A state law requiring importers to take out a license to sell imported goods amounts to an indirect tax on imports and hence is unconstitutional. 1 Likewise, a franchise tax upon foreign corporations engaged in importing nitrate and selling it in the original packages, 2 a tax on sales by brokers 3 and auctioneers 4 of imported merchandise in original packages, and a tax on the sale of goods in foreign commerce consisting of an annual license fee plus a percentage of gross sales, 5 have been held invalid. On the other hand, pilotage fees, 6 a tax upon the gross sales of a purchaser from the importer, 7 a license tax upon dealing in fish which, through processing, handling, and sale, have lost their distinctive character as imports, 8 an annual license fee imposed on persons engaged in buying and selling foreign bills of exchange, 9 and a tax upon the right of an alien to receive property as heir, legatee, or donee of a deceased person 10 have been held not to be duties on imports or exports.