The Postal Reorganization Act of 1970 was a law passed by the United States Congress that abolished the then United States Post Office Department, which was a part of the cabinet, and created the United States Postal Service, a corporation-like independent agency with an official monopoly on the delivery of mail in the United States. President Richard Nixon signed the Act in law on August 12, 1970.
The legislation was a direct outcome of the U.S. postal strike of 1970.
The first paragraph of the Act reads:
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.
The Postal Reorganization Act (at 39 USC 410(c)(2)) exempts the USPS from Freedom of Information Act (FOIA) disclosure of "information of a commercial nature, including trade secrets, whether or not obtained from a person outside the Postal Service, which under good business practice would not be publicly disclosed".
This summary is from Wikipedia.