A Constitutional Amendment is extraordinarily difficult to pass, and that’s why the U.S. Constitution has only been amended 27 times. The most recent Amendment — XXVII in 1992 — took 203 years to be ratified.
This definitely isn’t my area of expertise, but I would think running the postal service as a business — particularly today — would be ideal. I have no idea how to do that, but I think eliminating Saturday delivery is a start. Seems to me that FedEx and UPS have an idea how to do it and make a profit. I know there are major differences, but I would imagine that’s a place to begin.