I work at the USPS Main HUB in IND. Numerous airlines fly postal mail. Ryan, Emery, KittyHawk, Amerijet, Capital Cargo, Gemini, Evergreen.
Cities served from IND hub...DFW, SAT, GRR, LAX, SEA, BOS, IAD, MCO, GEG, PDX, OAK, PHX, MSP, SLC. I may have missed a few.
There are other point to point flights. As well as the USPS WNET system which is flown by all KittyHawk aircraft out of Mather Field in Sacramento.