"This says they were still worried about those Port Revenues"

Most certain port revenues (money) were a large part of their concern. Charleston Harbor was the second largest (in terms of trade) port on the east coast of the United States. Charleston in these terms was second only to New York harbor in terms of revenue gathered.

Port revenues was in fact the "Income Tax" of that day. It was one of the biggest (if not the biggest) direct sources of Revenue for the US Government allowed under the Constitution at that time. The growing trade in Cotton going to Europe and the goods being purchased in Europe and not from the northern manufacturer was a large part of why tariffs were place on those European goods coming into Charleston. More money!

That trade through Charleston was a very big apple to lose. At one point it was even discussed in some of their letters, if it would be reasonable to allow South Carolina to go ahead and secede so long as the revenues could still be collected at Forts by the federals. I don't right now remember where I saw that, whether it was between Lincoln and some of his associates. Or brought up during the "peace negociation" between the Confederate delegates and Lincoln in February 1861.

In any case that loss of revenue would have been a severe blow to the operations of the Federal Government in Washington. With its very limited abilities under the Constitution to raise monies a new form of Taxation would have to be found. Well today we have the Income Tax, which Lincoln and his associates came up with to pay for the War.

By the way while the Income tax was used in history (especially England) well before and after the War for Southern Independence, I wonder why it took an Amendment (16) to the United States Constitution in 1913 to define its legal status? After all wasn't the American Revolution fought as much about being overtaxed as it was freedom? Could it be that the Income Tax was Illegal before that time under the Constitution? It wouldn't be the first time that Lincoln and others threw the Constitution in the garbage can.