If that is the case, Dr. McWhiney taught you some very bad history. Ft. Sumter was never a customs collection point. In fact, Ft. Sumter was never really an operating military base. It was sill under construction when Major Anderson evacuated his small garrison from an indefensable position at Fort Moultrie.
The Customs House was in downtown Charleston, and was already siezed by the Confederates.
Dont' let my memory disparage Dr. McWhiney. However, I have run across several others saying the same as I was trying to research the issue. Regardless, Dr. McWhiney and Dr. Williams are great sources of objective history on the subject. If you want to know the truth, no matter what I say, I recommend them to you.