The USPS has a monopoly on using letter boxes. They're not allowed to go door-to-door with mail like the USPS does, period. Not to be so crude, but you don't get it. They charge $6 a letter because they have to independently deliver each package. They can't go door to door like the USPS does. Obviously, it costs a lot less per letter if every 12 steps you drop off another 10 letters as opposed to dropping one letter off, getting in your truck, and driving two streets over to drop the next. It's a scale economy---as FedEx delivers more letters, the average cost per letter decreases, making the service cheaper and more profitable. Letter delivery is a scale economy. It takes relatively similar infrastructure to deliver 100 million letters as it takes to deliver 100,000. But only one entity---USPS---is able to take advantage of the scale economy in letter delivery.
Why is home delivery (not the cost of the paper) of the Wall St. Journal more expensive than the Post Gazette? Because it takes A LOT more delivery people to deliver 1000 Wall St Journals because they're scattered all over the delivery area, while a Post Gazette delivery stops at nearly every house along the route.
If you allowed other businesses to use mailboxes (and got rid of USPS), the quantity of letters they deliver would soar, they would consolidate package and letter delivery and streamline routes.
I can't believe that people think USPS some how magically figured out a way to sustain a business without government subsidies by delivering letters, but somehow the logistical giants UPS and FedEx couldn't figure out a way to do it.