"Friend this letter is a long time over due. Hope you get it. Heard you was in Riverton. Im coming thru on the 24th, thought Id stop and buy you a beer Drop me a line if you can, say if your there." [story]
Jack and Ennis had to worry about interception of their messages to each other. By using postcards, they could hide in plain sight, using codes like 'buy you a beer,' 'fish will be jumping,' etc.
In the story, Jack's first message to Ennis was a "genral" delivery letter - later changed to a postcard. However the message still refers to itself as a letter.
"Friend this letter is long over due. Hope you get it. Heard you was in Riverton. I'm coming thru on the 24th, thought I'd stop and buy you a beer. Drop me a line if you can, say if your there." [screenplay]