“The warehousing act also benefited New York more so than Southern ports”
When you have ten times the capacity for ships, the act will have a disparity outcome. New York port could handle more ships than 90% of all the Southern port combined. Every Southern port with a U.S. Customs house had bonded warehouse.
Why did Northern business profit from mail contracts, A whole hell of a lot more people lived in the North to receiving mail. When a ship dropped off a load of mail in Charleston or Wilmington, Southern railroads were paid by the Federal Govt to transport that mail. There was nothing in U.S. Law that prohibited any Southern shipper contracting to transport that mail from North to any Southern port.
“And buying such from England or Europe was prohibitively expensive”. Tredegar in Richmond VA bought its raw materials for Pennsylvania and New York. The were shipped by rail. Tredegar was a very successful business. If it had not existed (despite your opinion that a Southern industry could not compete against a Northern Industry) The South would have lost 50% of their artillery pieces, the CSS Virginia would have gone without iron plate and most Southern railroads would have gone without rail.
“Toss out the law that prohibits the carrying of cargo on foreign built or foreign crewed ships between ports”
Without that law, Southern coastal shipping would have died due to competition from the Brits and the French
The reason there was no advantage to doing so was because of the navigation act of 1817. Charleston was as you said, much smaller, and could not handle traffic like New York, but this is self reinforcing condition. If it can't get the traffic, it can't grow, and if it can't grow, it can't increase it's capacity. The navigation act made the costs of going to Charleston the same as going to New York, and it was not only 800 miles further away, it didn't have the facilities to handle great quantities of shipping anyways.
Getting rid of the large tariffs, and getting rid of the ban on carrying cargo between ports, and it is instantly profitable to sail to Charleston, sell part of a cargo, then sail to Pensacola, and sell another part of the cargo, and then sail to Mobile, and sell the rest.
Instant money for Southern ports. Greater profits for European traders.
The money streams would have deflected from New York, and gone to the same places where the American export cargoes originated. The New York middlemen would have been cut out.
"...the mask has been thrown off and it is apparent that the people of the principal seceding states are now for commercial independence. They dream that the centres of traffic can be changed from Northern to Southern ports....by a revenue system verging on free trade...."Boston Transcript, March 18, 1861.
A year or so ago I read an article from some Charleston newspaper shortly after South Carolina seceded. It was full of stories about the massive building boom going on at the time, and how all the warehouse space and hotels in Charleston had been overwhelmed with customers. The city was going like gangbusters trying to build new warehouses, hotels, and other industries.
Charleston would have grown quite a lot, and it would have all come at the expense of the very wealthy people in New York who backed Lincoln's election to the Presidency.