Of course not. The USPS is not part of the government.
Well...USPS is legally defined as an "independent establishment of the executive branch of the Government of the United States," (39 U.S.C. § 201) and is wholly owned by the government and controlled by Presidential appointees and the Postmaster General. As a quasi-governmental agency, it has many special privileges, including sovereign immunity, eminent domain powers, powers to negotiate postal treaties with foreign nations, and an exclusive legal right to deliver first-class and third-class mail. (from wikipedia)