I found this little nugget at a flea market last summer, but I never got around to scanning for the antique mall/garage sale/flea market thread.
I was looking through a box that primarily contained sci-fi/fantasy art magazines when I came across this (the post-its were not included, just trying to maintain some privacy for the original owner):
The return address sparked my interest, so I flipped it over:
And finally, the prize inside:
I'm not sure how often these things turn up, but I was very happy to find them with the original mailer. It would have been nice if a thank you letter was included, or something on Mirage letterhead, but it's still a nice grouping.