99% of my junk mail is sent via USPS. Junk mail actually gets a better rate than regular 1st class mail. It's something like 20 cents for marketing mail vs 50 cents for a piece of regular mail. In exchange I think the marketer has to do some presorting of their own, and they lose the ability to do things like forwarding or return to sender.

USPS actually has a monopoly on mailboxes. It's illegal for FedEx or anyone else to put a letter/package in your mailbox, although I don't know how much that's enforced.