What do you suppose would have happened to Confederate efforts to gain recognition from Great Britain if Robert Barnwell Rhett (Author of the document you linked) had pointed out that slavery was the reason for rebellion?
I find this to be a much more plausible explanation why there are so many documents and statements by Southerners themselves addressing the need to protect slavery against the threat of a Republican administration, and so precious few that you can post stating purely economic reasons having to do with tariffs. Even he addresses his document to the other slaveholding states, not the other economically deprived states.
This is where so many people get misdirected. Tariff's were the lesser part of Northern angst. The far greater threat was the flooding of European goods throughout the United States, thereby displacing the production of the monied industrialists.
Not only would they lose their shipping, banking, insurance, warehousing and tariff collections, they would find customers would gravitate away from their products because the stuff made in Europe was much better, and now much cheaper.
The people of that era were very money conscious because it was so hard to obtain money. 30% or 45% extra cost would have encouraged people to look elsewhere, and that elsewhere was European imports coming in through the Confederacy.
It's about money. All else is noise and trash talk.