Every once in a while they will send it out without a prepaid envelope, but hang in there they will eventually send it out with one because their response rate is so low without prepaids.
Make sure that your name, address, offer code, etc. are not included in your return bonanza. I routinely put the “supplemental health insurance” stuff in the AARP envelope. A lot of the AARP stuff I put in the supplemental health insurance or other “free” offers prepaids.
Since I have informed them repeatedly to stop sending their crap, I don’t feel the slightest bit guilty about stuffing their envelopes.
Don’t worry about me accidentally sending back identifying information. I’ve gotten offers like this in the past from the Sierra Club and other lefty organizations including my college alma mater. My MO is usually to print out something attacking their organization (preferably multiple pages) and fill the envelope with that.