Interesting.. the same section banned any limitation on the importation of slaves... at least that clause had a sunset date. Both clauses were basically: "don't touch our cash cow".

The price of consensus

Looking at that clause, it appears that it's only unconstitutional if the individual states do it. Doesn't say anything about the federal government or Congress. Or am I reading it wrong?

There are two clauses that ban export tariffs. One applies to states, the other to the Feds.