Want FedEx to deliver a letter for you? They'l be glad to do it--for about twelve bucks apiece.
This may be why FEDEX doesn’t deliver mail.
Article I, section 8, Clause 7 of the United States Constitution grants U.S. Congress the power to establish post offices and post roads. The Federal Government has interpreted this clause as granting a de facto Congressional monopoly over the delivery of mail. According to the government, no other system for delivering mail - public or private - can be established absent Congress’s consent. Congress has delegated to the Postal Service the power to decide whether others may compete with it, and the Postal Service has carved out an exception to its monopoly for extremely urgent letters.
By law, only the USPS can deliver mail. Period.