I just make the back of mine look like a regular postcard or somewhere online there is a template for a postcard that you can print on to fabric via your printer. I did have it bookmarked but lost all that when I bought a new laptop.
I sent one to a person here on the board with a 44 cent stamp on it and dropped it in the mailbox and it was delivered with no problems. If it's really embelished, I would send it in an envelope.
Suzanne