Surprising though it may seem, with some limited exceptions, it's illegal for a private party to deliver a letter to another private party without paying the US Postal Service.

The term to search for is "Private Express Statutes": https://en.wikipedia.org/wiki/Private_Express_Statutes.

This article (titled "Universal Service and the Postal Monopoly: A Brief History") goes into more depth: https://about.usps.com/universal-postal-service/universal-se...

Besides the monopoly on letters that others have explained the USPS also has a legal monopoly on the use of your mailbox.

Only usps can carry first class mail, or “letters”.

Google or Bing help a lot.

See eg https://en.wikipedia.org/wiki/United_States_Postal_Service#C...