Take up that argument with the general public, not me, or as we used to tell folks over the phone (but never in writing) take the issue to your Congresscritter ~ we just work here. (although I'm long retired, it hasn't changed much).
“BTW, USPS’ monopoly, though seemingly granted by the Postal Monopoly Act, really arises out of action of the United States Constitution wherein Congress is given sole authority to establish post offices and post roads.”
Wrong. I have the US Constitution in front of me. Article I, Section 8, Paragraph 7: “The Congress shall have power to establish post offices and post roads.” I don’t see anything in that clause — or anywhere else in the Constitution — granting to Congress exclusive or sole authority to do so; nor do I see anything in the Constitution denying to states or individuals the right to establish post offices and post roads.
What gave USPS the sole right to deliver first class mail was the Postal Act of 1845 - the “private express statutes” — that explicitly prohibited private companies from carrying letters. . . the operative word, “letters”, being anything the USPS decided to define as such. Here’s a Cato Institute paper on this:
http://www.cato.org/pubs/journal/cj5n1/cj5n1-8.pdf
Several attempts in the past have been made to compete directly with US Post Office in the delivery of first class mail. An interesting one was made by the 19th century libertarian and abolitionist Lysander Spooner. That story is here:
http://www.digitaljournal.com/article/271139
When USPS waived its monopoly on “urgent delivery”, the door was opened for competition in that area from private companies like FedEx and UPS. As for 1st class mail delivery, it is technically possible to send a 1st class letter via FedEx or UPS, but they are LEGALLY REQUIRED to charge a certain minimum price that is higher than it ordinarly would be — a price that makes such deliveries uncompetitive with the subsidized price provided by USPS.