Depending on how you calculate it, $2 million in 1857 equates to hundreds of millions today.
So let's note that first, what you here call "subsidies" or "underwriting", in other posts you've admitted were merely payments for services, such as transporting US mail.
Second, given the nature of Federal government at the time, we don't expect that much, if any, of that money was outright grants -- no "free money".
Third, Federal contracts were supposed to be awarded on the basis of competitive bids, and the fact that you've listed not just one monopolizer, but rather six different carriers strongly suggests plenty of competition.
Fourth, from the names we might well suppose that shippers listed were Northern owned & operated, though the Charleston Line and George Anson are more associated with Charleston SC.
Bremen Line and Havre Line sound more foreign owned than US.
Your numbers do not suggest which shippers carried mail to & from such major Southern ports as New Orleans and Baltimore.
But here's the important point to remember: in 1860 80% of US citizens lived in Union states and 90% of US city dwellers lived in Union states.
So we would not expect Southern mail carriers to add up to more than 10% to 20% of the total.
And one might well suppose that with so little mail to carry, Southerners would focus their economic attention elsewhere, such as on growing highly profitable cotton.
Finally, the nature of politics being what it is, we can presume that whatever benefits Northerners derived from Federal spending, Southerners would demand & receive their "fair share" in some other activity.
And I chuckled a bit at this (bold emphasis mine):
I t is to be expected that when the major portion of the people of a country dwell near the seashore, and hear the “ call of the sea, $ a very great interest will be taken by the general public in shipping affairs . Therefore, it was natural that the colonies were scarcely well settled before they began to endeavor to build up their shipping by discriminations at the expense of one another or of alien carriers . A number of the colonial charters authorized the levying of discriminating duties . Virginia seems to have been the first, in 1 63 1 , with a duty of two and one half per cent on goods imported by foreign subjects, and five per cent on all goods imported by foreigners , the latter goods presumably in foreign bottoms .
It goes on with a recitation of the coastal states and how & when they entered into the competition for shipping. The steps that were taken by the states were against foreign interests, not domestic ones, and all were invited to the party.