[riffic]

wasn't always that way. At one time the predecessor agency to the USPS was a cabinet-level department:

https://en.wikipedia.org/wiki/United_States_Post_Office_Depa...

There is Constitutional authority empowering Congress "To establish post offices and post roads" (https://en.wikipedia.org/wiki/Postal_Clause)

We also used to have a limited form of postal banking here:

https://en.wikipedia.org/wiki/United_States_Postal_Savings_S...