During one of my searches online, I stumbled across this account on flickr by Danielle Maret. She makes art on post cards and envelopes that's so amazing it's like you are dressing up your mail with them for someone special. She uses paper from books, cutting from magazines, patterned pieces of paper, tags, and maps in her illustrations that makes each envelope a unique piece of art.
You can visit her profile on flickr from the link above to see more of her work. It's a shame that she hasn't posted any new work in a while. If you know where can I learn about what's she currently working on then let me know.