My guess would be February or March 1861 after the state announced their secession and the Confederacy was organizing itself.
But according to Treasury Dept. data, tariff deposits dropped to $41.5 for calendar year 1861.
Figures would be for fiscal year, which at the time ended June 30th. So we're talking about a drop in revenue of about 23 to 25%.
For calendar year 1862 and forward, data includes the new variables of raw war material imports, and foreign goods now having to be imported as a substitute for Southern goods no longer available.
I highly doubt that the government would tax the very goods they need to fight the war, and other than cotton what did the South provide to the North that was taxed?
You are moving into speculation which is of no interest to me.
About Southern commerce with the North, they were buyers of Cotton and food.