You can use Fedex, UPS etc to ship anything from anywhere to anywhere. Nothing is stopping you from using fedex to ship your christmas cards as well. I assume people stick with USPS for simple letters because its cheaper and easier, not because of any enforced monopoly.
There’s a monopoly on residential letters, and a monopoly on mailbox access:
https://en.wikipedia.org/wiki/United_States_Postal_Service#U...
> You can use Fedex, UPS etc to ship anything from anywhere to anywhere.
You actually can’t, there are very specific rules for what you can use a non-postal carrier to send to which addresses and at which prices. Which is why you don’t see general, non-express, or cheap letter delivery from third-party services.
The USPS was created in 1971, which is not the 18th Century.
(Heck, even the monopoly rules that originally applied to the Post Office that now apply to the Postal Service were first established in the 19th, not 18th, century.)