"73%" is a bogus number, the real number is roughly 50% of US exports were Deep South cotton, everything else claimed as "Southern products" was produced in whole or in part in Union states.
The value of cotton exports in 1860 was approx. $200 million.
Southern cotton growers were typically paid for their products, sometimes ahead of the harvest, by middlemen called "factors" who then typically arranged for "ginned" cotton to be transported to a dock for export.
So before cotton ever left his property, the cotton grower was paid.
Who were these money-men "factors"?
Some were Southern bankers, some were Northerners, some, doubtless, represented major European buyers.
And once the cotton grower is paid for his products, the methods & routes of transportation to foreign (or domestic) manufacturers is utterly irrelevant to the grower.
But, as it happened, in the vast majority of cases, cotton shipped directly from a major Southern port -- i.e., New Orleans or Mobile -- to its customers in Europe or New England.
And once the ship-owners unloaded & sold their cotton, then, naturally, they looked for paying cargoes for their return trips.
These returning cargoes included the raw materials and luxury goods needed by American businesses & wealthy people.
And the largest concentration of such businesses & wealthy people was in New York City, or could be quickly & cheaply reached from NY via America's railroads & river steamboats.
So NY is where those return ships stopped first & sold their cargoes, imports which then went into special bonded warehouses, which by law did not have to pay import duties until sold & shipped elsewhere?
The total value of those imports was circa $400 million in 1860.
So how many of those $400 million in imports ended up in the hands of Souther cotton growers?
Answer: very little, a tiny percentage, and the reasons are obvious.
Nearly everything Southerners imported, they "imported" from the North.
And those "imports" from the North were neither raw materials nor foreign luxury goods, but mainly basic manufactured cotton & woolen clothing, shoes, iron stoves, hats, farm tools & railroad iron, etc.
Bottom line: in 1860 Southern cotton exports of $200 million amounted to roughly 5% of the US total GDP of $4.4 billion.
The entire Southern economy amounted by roughly 15% of the US $4.4 billion GDP, about $650 million.
And these numbers were instantly removed from the US economy in 1861, resulting in serious economic issues, not least because New Orleans was closed to Union traffic.
But the Union economy did quickly adjust & recover and by war's end had more than doubled.
And of course, none of this had anything to do with events at Fort Sumter, SC, or the Confederacy's Declaration of War on the United States, May 6, 1861.