[swmobuffalo]

“back in the 1930s. However, USPS didn’t exist then!”

This little factoid says differently.

Oldest Post Office is in Castine, ME. This office has been in continuous operation since 1814.

http://www.usps.com/communications/newsroom/postalfacts.htm

http://www.usps.com/postalhistory/postal_service_begins.htm

[muawiyah]

You'll find, if you bother to look, the entity known as the United States Postal Service began operation on July 1, 1971.

See: http://www.usps.com/history/history/his3.htm#SERVICE