If all those goods were headed to Northern customers, then why would Northern ports be so upset by an independent South charging a lower tariff? All the North had to do was charge a tariff wherever the goods crossed the border. That would nullify any advantage the Southern ports might have had by lowering tariffs.
On the other hand, if a sizable portion of the goods were ultimately for Southern customers, then Northern ports (and Lincoln) would have been upset over revenue lost to them. This makes more sense to me.
Cotton revenue would have generated enough money for Southern planters to import goods. Take a tour of some of the surviving plantation homes sometime and see all the imported finery.
It violates the Constitution, for one thing. All tariffs must be uniform throughout the states.
"All" those tariffs were -definitely- collected in northern ports. The southern rail system was designed to ship cotton out, not ship goods in.
Walt