Not sure if they have to pay any significant amount for the emails, but the business reply envelopes they send out they do have to pay return postage on, based on weight. So if someone were to stick lead bars in the envelopes, or some other creative method for costing them cash, that would be a better plan.
Just be aware that there is usually a bar code on the bottom of the return envelope so they can ID who replied, so if you were dumb enough to send an envelope full of talcum powder they would probably find you.
I know this because I once sent about 50 loose pennies in a Brady Campaign business reply envelope, hoping the weight would cost them cash, and that the pennies would jam up their automatic letter opening device and maybe the shrapnel would take a couple of the bastards out, but instead I got a letter back thanking me for my generous donation even tho I only loaded it with the pennies and no return slip or other identifying info.