Methinks USPS should revert to the library model: office in every town, with office-to-office delivery guaranteed as a Constitutional obligation. The door-to-door thing is a step beyond what’s required, and is a relatively recent phenomenon.
Just to stir the pot: one could argue that due to technological advances the Constitutional “post office” clause could cover email as well, requiring USPS provide at least basic email service.
As I read it, the constitution only authorizes congress to create a federal post office, it did not require it.