The United States Post Office is a real-life chain of post offices in the United States. Although a Postman who works for the office has been seen in several episodes, the actual post office itself only appeared in the Season Fifteen episode "Crack Baby Athletic Association".
Craig Tucker and Clyde Donovan go to the United States Post Office to ask the postmaster Marty where Slash lives. The postmaster reveals to them that Slash is make-believe, but his co-worker tells him not to tell them that and that it is their parents' responsibility to tell them that Slash is not real. Clyde then calls his parents to ask them if Slash is truly make-believe.