Snail mail spam

I really hate getting flyers and crap that I just throw out in the mail. So this time I've decided to do something about it. I'm sending that lovely prepaid envelope back to them with some goodies inside. Some paper from my other mail, some plastic bags, some cardboard from a drain plug, a slurpee lid, a mini eggs package, two tart tins, two batteries, and an old Tide to Go pen. I taped it all up in that envelope and will mail it soon. I'm not sure how prepaid envelopes work. I'm thinking they would pay for the ones that they get back... so this one is going to be pricey because it is heavy.