And until very recently they were the main money-making department of the federal government, funding other departments. Think of what else the government provides as a true service that makes money? None I can think of.
The Postal Service as USPS has been unionized since day one ~ and postage wasn't 1cent or 3cent for a very long time before that agency was established.
How old are you? Are you still sitting out there on the front steps of the Philly main office?