Read it. Then consider who is the author of USPS troubles.
The only mention of postal services in the Constitution appears in Article 1, Section 8: The Congress shall have Power To establish Post Offices and post Roads; That’s it. Not has to, has power to.
The Congressional mandate for universal postal service and the creation of a government protected monopoly is a legislative choice, not a Constitutional mandate.
But, don't take my word for it. In a footnote to its 2014 report on postal finances, the Library of Congress' Congressional Research Service had this to say:
The USPS often is mischaracterized as a quasi governmental or private entity. It is neither. The USPS is a government agency that was created by Congress to achieve various public purposes. Federal law defines what products and services the Postal Service may offer. Additionally, the USPS’s employees are federal employees who participate in the Civil Service Retirement System, the Federal Employees Retirement System, and the Federal Employees Health Benefits Program.The Post Office suffers from its slavery to politicians and union potentates who put their own interests first. The result for the USPS is 'corporatism without the benefits.'
The pretense that the USPS is a not a government agency is a crony-capitalist obfuscation. The USPS has the WORST of both worlds, the congress controls what they can do and must do, yet forces them to do business whether it’s profitable or not. Congress micro-manages USPS real estate and employment rules and also sets the price of USPS’s mandated services.
Contrary to President Trump's narcissistic whining (which is really about WaPo coverage), Amazon is not the problem. A little fenced off corner of The Swamp is the problem.
But acknowledging that won't help his poll numbers or justify his poorly disguised animus toward a man who probably could benefit from a dispassionate thumping.