From Wikipedia: United States Post Office Department

One of the early executive departments, it is now a corporate like independent executive agency. These are sometimes called quasi-governmental agencies. 

- Click here for the entry.

The United States Post Office Department (USPOD; also known as the Post Office or U.S. Mail) was the predecessor of the United States Postal Service, in the form of a Cabinet department, officially from 1872 to 1971. It was headed by the postmaster general.

The Postal Service Act, signed by U.S. president George Washington on February 20, 1792, established the department. Postmaster General John McLean, in office from 1823 to 1829, was the first to call it the Post Office Department rather than just the "Post Office." The organization received a boost in prestige when President Andrew Jackson invited his postmaster general, William T. Barry, to sit as a member of the Cabinet in 1829.[1] The Post Office Act of 1872 (17 Stat. 283) elevated the Post Office Department to Cabinet status.

- Click here for a link to the Postal Service Act, which was passed by the Second Congress.

These might be useful as well: 

- Post Office Act (1872).

- Postal Reorganization Act of 1970.

- United States Postal Service.