You should send any post-paid envelopes back to them, at the very least.
That's what I do. I also tear their propaganda in half and enclose it to increase the weight, after carefully removing my name/address.
AARP is nothing more than an arm of the DemonRat party. I always knew that and never ever considered joining. I always thought that fact was common knowledge, but apparently not, as some are waking up to what the organization is really all about now.