Ian Urbina of the New York Times wrote on marketing messages (No Need to Stew: A Few Tips to Cope With Life's Annoyances):
"When subscription cards fall from magazines Andrew Kirk is reading, he stacks them in a pile at the corner of his desk. At the end of each month, he puts them in the mail but leaves them blank so that the advertiser is forced to pay the business reply postage without gaining a new subscriber."
Thank You! What a lovely idea.
So far I've sent in three empty business-reply envelopes. It feels GREAT.