Article 1, Section 8, Clause 7: To establish Post Offices and post Roads;
Until 1982, the postal service was a tax-payer funded organization and postage stamps were considered a form of taxation. Under Federal law, the postal service is allowed to run a deficit and unless the United States files for bankruptcy, it can't go "bankrupt" or "go out of business". Comparing them to UPS is FedEx is just idiotic, but then again, look no further than the source (be it Wyldeman or Fox News).
"The United States Postal Service shall be operated as a basic and fundamental service provided to the people by the Government of the United States, authorized by the Constitution, created by Act of Congress, and supported by the people.
The Postal Service shall have as its basic function the obligation to provide postal services to bind the Nation together through the personal, educational, literary, and business correspondence of the people.
It shall provide prompt, reliable, and efficient services to patrons in all areas and shall render postal services to all communities.
The costs of establishing and maintaining the Postal Service shall not be apportioned to impair the overall value of such service to the people. (b) The Postal Service shall provide a maximum degree of effective and regular postal services to rural areas, communities, and small towns where post offices are not self-sustaining.
No small post office shall be closed solely for operating at a deficit, it being the specific intent of the Congress that effective postal services be insured to residents of both urban and rural communities. (c) As an employer, the Postal Service shall achieve and maintain compensation for its officers and employees comparable to the rates and types of compensation paid in the private sector of the economy of the United States."
http://us-code.vlex.com/vid/sec-postal-policy-19236133