Replies

Sherman Logan: "What Walter was probably thinking about in his 75% number was the value of exports.
For which the South did provide something along that percentage, mostly cotton."

This link from iowamark's post #25 above, shows that total 1860 US exports were $400 million.
Of that, this link shows total cotton exports were less than $200 million, meaning cotton in 1860 accounted for less than 50% of all US exports.

But the numbers also tell us that Southern US cotton accounted for 75% of all cotton produced worldwide, which was the key fact behind the Confederacy's "King Cotton" strategy.
And this link tells us about 40% of cotton shipped from New Orleans -- with 85% of that going to such European countries as Britain & France, just 15% to Northern US manufacturers.

Bottom line: in 1860 Deep South and Upper South whites totaled about 5.5 million or 20% of all US whites.
Yes, on average they were somewhat more prosperous than their northern cousins, and tens of thousands of plantation owners were very prosperous indeed.
So they could well account for a disproportionate share of imports -- perhaps 25% in total -- but there is no way they could be the purchasers of a majority, much less 75%, of US import duties.