Yes, they are indicated in the Constitution, and they must service each and every little town, and they must go to Congress to gain approval for rate increases, but they are not supported by any tax revenue.
This is not true, but a common debunked claim from the postal union. The postal service gets $18 billion per year in tax benefits including exemption from property taxes and vehicle registrations as well as below-market borrowing costs.
Yes, I was mistaken. Thank you for the clarification.
In a way, that makes the original complaint even more ridiculous because it is literally a business transaction between two private entities and a private intermediary.
http://fortune.com/2015/03/27/us-postal-service/
http://www.politifact.com/georgia/statements/2013/jul/24/ame...