Whenever I mail to fellow artists, friends and sometimes clients I try to add some form of original art on the envelope. I started doing this when I first got involved in "mail art". I loved receiving mail with original art on the "outside" of the envelope. In fact I have quite a collection of envelopes from fellow artists that I treasure deeply. I have some envelopes that almost every square inch is covered in some sort of doodle or message. Some artist's envelopes are highly collectable these days.
I've put a sample of one of mine on here. I usually do something with the water because of my name "Southshoreartist".
Just google "mail art" and you will find 100's if not 1000's of sites dedicated to the movement. Although I'm not active in the movement anymore I still like to do my part and "share" original art through the mail.