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.
Only articles imported from or exported to a foreign country, or
a place over which the Constitution has not extended its commands with respect to imports and their taxation, are comprehended by the terms
exports. 1 With respect to exports, the exemption from taxation
attaches to the export and not to the article before its exportation, 2 requiring an essentially factual inquiry into whether there have been acts of movement toward a final destination constituting sufficient entrance into the export stream as to invoke the protection of the clause. 3 To determine how long imported wares remain under the protection of this clause, the Supreme Court enunciated the original package doctrine in the leading case of Brown v. Maryland.
When the importer has so acted upon the thing imported, wrote Chief Justice Marshall,
that it has become incorporated and mixed up with the mass of property in the country, it has, perhaps, lost its distinctive character as an import, and has become subject to the taxing power of the State; but while remaining the property of the importer, in his warehouse, in the original form or package in which it was imported, a tax upon it is too plainly a duty on imports, to escape the prohibition in the Constitution. 4 A box, case, or bale in which separate parcels of goods have been placed by the foreign seller is regarded as the original package, and upon the opening of such container for the purpose of using the separate parcels, or of exposing them for sale, each loses its character as an import and becomes subject to taxation as a part of the general mass of property in the state. 5 Imports for manufacture cease to be such when the intended processing takes place, 6 or when the original packages are broken. 7 Where a manufacturer imports merchandise and stores it in his warehouse in the original packages, that merchandise does not lose its quality as an import, at least so long as it is not required to meet such immediate needs. 8 The purchaser of imported goods is deemed to be the importer if he was the efficient cause of the importation, whether the title to the goods vested in him at the time of shipment, or after its arrival in this country. 9 A state franchise tax measured by properly apportioned gross receipts may be imposed upon a railroad company in respect of the company's receipts for services in handling imports and exports at its marine terminal. 10