Then, explain this:
“The difference is so great between the tariff of the Union and that of the Confederated States, that the entire Northwest must find it to their advantage to purchase their imported goods at New Orleans rather than at New York. In addition to this, the manufacturing interest of the country will suffer from the increased importations resulting from low duties.“
if by buying you mean onesies or twosies then tariff is probably not a problem. If you are going to buy 40 tons of iron rail and ship it North you will be required to pay the tariff because you are importing a tariffed product into the United States. No different than if you bought that 40 tons of rail in Birmingham England and had it shipped to New York. The principal in both situations is that the source is outside of the United States. After secession New Orleans in in a Foreign country.