Constitution of the United States:
Of course, having the power to establish post offices is not a mandate to do so. And I suppose it follows that if the Congress has the power to create post offices, it also has the power to uncreate them.
The power of the Post Office is two-fold: it assures that the mail goes through and it ensures that the poorest of the poor are able to communicate with the government. By being a public, tax-supported activity the PO cannot (in theory) be shut down by strike, or priced out of the reach of some. The PO has been closed by strike once - in 1970
http://en.wikipedia.org/wiki/U.S._postal_strike_of_1970 . The NG was called out to deliver the mail. By being supported by taxpayer monies the cost of communicating with the government is kept within the reach of even the poorest citizen - well, in theory at least. Both First and Second Class mail is still subsidized at well below actual cost, although the fees for other services have not supported the total expenditures for many years now.
When the PO was restructured into the USPS, the quasi-, semi-governmental entity, it was supposed to become much closer to self-supporting. That has never happened, even with the continuation of their monopoly on delivery to those cute little boxes on the front of houses or at the side of the road, which is why they hand all that crap on your doorknob or stuff it between your screen and front door. And which is why UPS/FedEx can never deliver First Class mail, even if they really could do it faster and cheaper and better. Or why Alaskans can mail enough bricks to build a mansion for less than the cost of trucking them there from the lower 48.
Speaking of the power of the PO - a quiz:
At the intersection of Mass Ave, NH Ave, Conn Ave, 19th St, P St, and Riggs Ct in NW Washington DC you have the following vehicles approaching: on Mass Ave moving SE to NW the President of the USA and the Dutch Ambassador in a motorcade, on NH Ave moving N to S you have a fire truck with lights & sirens, on Conn Ave moving NW to SE you have an ambulance with lights & sirens, on 19th St you have the FBI and Secret Service in a running gunfight chasing robbers from the US Treasury, on P St moving from W to E you have the Pope and Mother Teresa (pretend she's still alive) on their way to Children's Hospital to save the life of little Susy via a miracle, and coming out of Riggs Ct is a mail truck. Which vehicle has the right of way?
Say it with me, class: the mail truck!
stay safe.