As the Brookings Institution reported, the USPS is run like a private business, but it isn't owned like one. Its status as an independent entity in the executive branch of the federal government was created by the 1970 Postal Reorganization Act. Before that it had been the Post Office Department, established in 1792, and was later changed to a Cabinet department in 1872. The first U.S. Postmaster General, appointed in 1775, was Benjamin Franklin, because, of course it was. The guy did everything at the beginning of the United States, besides darn everybody's socks.

https://www.grunge.com/277915/who-owns-the-post-office/

Well that’s interesting.

https://en.m.wikipedia.org/wiki/Postal_Reorganization_Act

More interesting:

The legislation was a direct outcome of the U.S. postal strike of 1970. Prior to the act, postal workers were not permitted by law to engage in collective bargaining. In the act, the four major postal unions (National Association of Letter Carriers, American Postal Workers Union, National Postal Mail Handlers Union, and the National Rural Letter Carriers' Association) won full collective bargaining rights: the right to negotiate on wages, benefits and working conditions, although they still were not allowed the right to strike.[3]

So they attacked the President with agitated unions through an official department.

Sounds like it was done as an emergency cancer surgery.