The Data and Marketing Association says American businesses sent 149 billion pieces of direct mail and nearly 10 billion catalogues in 2016.
https://www.washingtonpost.com/lifestyle/home/how-to-stop-ju...
This is just a counter-anecdote, but for every flyer on the door I get (once or twice a week), I get scores of pieces of junk mail. I assume you don't own a home, because that alone is good for 10 a week.
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.
All of the junk mail I've received has come via the USPS, however since this all started happening I have received almost none.
The drop-off of junk mail delivery has been both welcome, and a sign of troubles for the USPS.
The postal service might add a flyer to your actual mail delivery, but only if they're delivering to your address anyway.