‘the United States Government was given the power to to establish Post Offices but that does not provide a mandate that it must’
Precisely.
It can abandon providing postal service just as it abandoned providing post roads.
Today's manifestation is mostly financial ~ Interstate Highway system comes to mind as does AMTRAK!
Earlier we had US Highways (US 50 is an example) and CONRAIL.
Before that we had Abe Lincoln and his cronies giving away alternate sections in the Great Plains to the railroad construction and operating companies to "please build a Transcontinental Railroad for us".
TheNational Road and other "post roads" were built at federal government expense in the early days, and even CANALS received substantial federal largess (although the private and state component of the financial intrigue associated with canal building left many Midwestern states BANKRUPT).
Here's another manifestation of "post road" ~ see those postal trucks parked on the street any time of day wherever they wish? They can do that provided that is a "postal road". You don't want a "postal road"? Then you don't get mail delivery or transport on your street. That can be done as a private road or just an exclusive municipal or state road.
So, what would those be? Well, they have them at many airports ~ for example, USPS trucks do not have access to the main runways! UPS and FedEx do, but not USPS.
Those particular "roads" are reserved for specific types of vehicles (USPS doesn't own any planes) under certain conditions, but, as it turns out the whole scheme for collecting taxes and paying for construction of airport runways is done under the "post roads" clause.
The same clause probably serves to authorize subsidies for port facilities as well.
True enough, but it also prohibits anyone else from filling the gap, which is why you don’t see private postal roads.