You ignore the loss of income from sailing a ship to a Southern port with an empty cargo hold. That represents loss of money.
Sure, they could do it, but Northern ships didn't have to do that. They could carry cargo on every run and thereby make more money than their competition.
It was economically unfeasible.
Why would they have to sail with an empty hold? I posted numerous examples of foreign ships bringing goods with them to Southern ports, all taken from contemporary newspapers. Furthermore, foreign ships could bring passengers from Europe and return with goods from America.
To recap:
The reason for secession was the South’s desire to preserve slavery.
The trigger for secession was the election of Abraham Lincoln, which fed fear of the abolitionist foundation of the Republican Party.
The American Civil War began when units of the South Carolina Militia fired upon a Union fort in Charleston harbor.