The problem with eliminating the USPS is that the Constitution authorizes a postal service; and as I said, being a Constitutionalist, I’d strive for following the Constitution.
The DOD itself isn’t mentioned in the Constitution and could therefore be eliminated; the same with the Air force, NASA, the Coast Guard. If those entities do serve a valid federal function they should be folded into Constitutionally sanctioned organizations (ie Air Force -> Army Air Corp).