Wouldn't the tariff be applied once the goods were landed in the United States?
Upon secession, federal tariff offices shut down.
Yes, tariffs would apply above the City of St Louis, MO. DL’s theory is that the riverboats carrying the cargo would stop short of the U.S. border, discharge their cargos to hundreds of waiting wagons. These would then smuggle the goods into MO, KY and other points North. Thus avoiding the tariffs. There would be not legal way to bring those goods into the United States from the Confederacy unless the U.S. tariff was paid.