William O. Douglas,
Canton Railroad Company v. Rogan…
“ The evident policy of the Constitution is to avoid these burdens and maintain free and equal access to foreign ports for the inland areas. If the constitutional policy can be avoided by shifting the tax from the exported article itself to some incident such as carriage, unavoidable in the process of exportation, then the policy is a practical nullity. I think prohibition of a tax on exports and imports goes beyond exempting specific articles from direct ad valorem duties-it prohibits taxing exports and imports as a process. ”
