Please see my earlier post (#22) in this thread.
What would it take to completely shutter the USPS? Would an amendment be necessary because of what's widely known as the "Postal Clause"? Or is there some form of a "Sunset Provision" that could be applied retroactively?
Failing that, what measures could be taken to reduce the role of the USPS to that only minimally satisfying the letter of the Constitution?
It would take an amendment as the Constitution clearly states:
The Congress shall have power...
To establish post offices and post roads
Without an amendment, that power is part of Congress.