Business Reply Mail
The Centennial Society has a few suggestions, and they've found a handy way to disseminate the playfulness.
"This small, sixteen-page pamphlet is produced to put inside the postage-paid, business-reply envelopes that come with junk mail offers. Every envelope collected is stuffed with the pamphlet and mailed back to its original company."
Business Reply Mail via The Centennial Society, famously of the Fallen Rapper Pez Prototypes.
Thanks to gerf