WAHTS THIS MOD CALLED IMMA NEED DAT!!! (sorry for not uploading a image, i am just asking a question)
The dev team is currently developing them, but at this time they are not yet for sale, though they are working on it - keep watching the devblog and forums for updates on that! I got mine as a prize from the Community Mission Routes Contest.
I wonder if they're planning to sell those postcards as part of a (limited) physical release OT: Grey Gum