YOu have no idea how hilariously subsided agriculture is in the USA.

This (mostly) isn't a bad thing, it buffers farms and ranches and orchards against year to year fluctuations and ensures a consistent domestic food supply which is damn useful. But food would cost a lot more and the cost would vary a lot more from year to year and month to month without that.

Also competition is only effective market control when the barrier to entry is lower. it's not that hard to start up a grocery store.

A national postal service the can deliver a letter from bug fuck no where louisiana to bug fuck nowhere alaska? That's going to cost you billions in start up costs, and you're going to lose money. Fedex and USP both cost way more more than the USPS and can't deliver as efficiently. They're good at what they do, but there's a reason it'll cost you 4 bucks to send a small package via USPS priority mail and 14 bucks to send the same package via UPS ground. They're freight companies that do home delivery, not a mail service.

You can go plant an apple tree if you think the price of apples are too high. If the cost of mail is too high, you're entirely fucked. Without the USPS, the cost to mail anything under 10 lbs would double overnight, just because UPS etc can't afford to run the kind of scale of operation to get close to the USPS's prices. And without the competition from the USPS driving it down, they'd have zero reason to not just raise the prices after that. And if that happens the price of everything increases because modern society does not function without the mail.