Mail to the west coast was heavily subsidized by Washington. Overland costs were so incredibly high that it was about as cost effective to transport mail by steamer to Panama, move it over the isthmus, then by steamer to California and Oregon. A large fraction of the west coast mail involved the business of the federal government - particularly the army - so it was deemed in the national interest to subsidize its timely transport - and both Whig and Democrat administrations had supported a heavy subsidy - and put their effort to squabbling over the overland route's location. It could be said then that a national consensus had been reached that postage rates to the west coast were to be kept well below costs.
Obviously to most - well, maybe not you - climate argued for sending overland mail on the southern route - for year-round service reliability. Equally obvious, steamships have to go south to reach Central America. Steamship companies already operating in the South had a clear advantage on bidding on the routes. The figures cited show that the big picture for revenue and expenses included west coast costs in the Southern column. As the transport of mail was yet another sectional controversy, it is reasonable that Stephens included California and Oregon in this way as their congressman and senators were heavily Democrats.
I know this spoils an opportunity for you to demagogue, but maybe others will appreciate it. And maybe the newcomers will get a look at your irresponsible or uninformed methods of argument. Now, please inform us where you ran across this little gem. I'm sure you didn't look it up yourself. (If I had to guess, I'd say McPherson, but that is just a guess. It's typical of his intentional distortions).