The United States Postal Service (USPS) is an independent agency of the executive branch of the United States government responsible for providing postal service in the US.
The United States Post Office (USPO) was established in Philadelphia under Benjamin Franklin on July 26, 1776 based on the Postal Clause in Article One of the United States Constitution. This empowered Congress “To establish post offices and post roads.” In 1792, it became the Post Office Department and was part of the Presidential cabinet and the Postmaster General become the last person in the United States presidential line of succession. In 1971, the department was renamed the United States Postal Service (and reorganised as a semi-independent agency of the federal government). The Postmaster General is no longer in the presidential line of succession. See Wiki for further historical background.