In the movie, the only post office from where Ennis sent and received mail was the one in Riverton, Wyoming.
But, in the book, Ennis was not even in the same county as Riverton, Fremont, when he bought the Brokeback Mountain at Higgins Gift Shop. Ennis was working on the Stoutamire cow and calf outfit at Signal when he got the "Deceased" stamped postcard about the November meeting returned.
At least the post office in the movie also has a red brick front like the real one.
I just thought of why there are so many differences between Annie Proulx's story and the one which is told by the movie, those involved with the movie used "artistic license" in their interpretation/adaptation of the original short story.
I used "artistic license" when I drew and/or painted scenes, landscapes and/or buildings. People who had been to the location from where I got my inspiration knew exactly where the scene was from and some would even swear that what I composed was a picture perfect copy of the actual place. But, I would make changes just enough to make things look more pleasing to the eye.