Ships bringing imports to the US went to the ports that provided the best access to consumers. That was New York (also Boston, Philadelphia, Baltimore). New York was valuable because the city's railroad network made a vast area with much of the country's population easily accessible. New Orleans and other Southern cities didn't provide access to as large a pool of consumers, so imports weren't delivered there.
An article here recounts the problems that led to Charleston's decline as a seaport:
A healthy port, after all, must rely on a thriving hinterland and an effective transportation network to feed it. Hemp grown in eastern Kentucky, for example, was used to bundle southern cotton. But to reach South Carolina, the hemp had to be sent north by rail to the Great Lakes, by barge to New York, and then by coastwise vessels to Charleston. In this era, “New York has a hinterland that stretches west to Indiana” and into the South itself, says Scott Reynolds Nelson, a historian at the College of William & Mary. “South Carolina has a hinterland that doesn’t get to Tennessee.”
You keep repeating that foreign shippers could have carried goods more cheaply and that US shippers raised prices because of the Navigation Act, but you don't provide any real evidence. Competition between domestic shippers would have kept the cost of coastal shipping from going too high.
Your idea that there was some Northern monopoly of coastal shipping runs aground when we look at the domestic slave trade. So far as I've been able to find out slave traders in Virginia had no trouble renting space on or chartering or even buying and operating ships to send slaves to New Orleans for sale. Article about the brig Uncas. More about the four ships owned by Franklin & Armfield.
There was no Northern monopoly of coastal shipping or of transatlantic shipping. The problem was that there was no Southern market for any foreign goods as big as the Greater New York market was, or as big as the market for slaves in the boom days of cotton growing on the Mississippi was. Also, Southerners didn't put as much effort into shipping as Northerners did. It wasn't a priority for them.
This message appears to be one of those such times. With most people, I can just breeze through the discussion with references from my memory, or the few links I can still remember how to fine, but with you, it's always plodding dog tiring work to give you back a good answer.
You kinda take the fun out of having these arguments. :)
Your response will have to wait till I get bored enough with the replies, and then I will go try to look up the information.