A U.S. Mail post office in Ganton.
U.S. Mail is a post office located in Ganton, and Willowfield, in San Andreas. It is, however, inaccessible to the player at all times and is not featured in any mission, as it's just part of the scenery. But if the camera is tilted in the right way there is a rabit hole of the inside which features a room similar to the horse betting rooms.