Re: The thing is... it's nothing new.
It's what the sender's paper recycling bin is for when their litter's been posted back to them.
Mix'n'match spam and prepaid envelopes. Keeps Royal Mail solvent and the spammers out of pocket. I'm sure Anglia Windows would like an MBNA credit card and MBNA would like some new-age meals on wheels and a few Farmfoods vouchers¹, so my handy pass-it-on service fills a gap in the market.
Am I evil? Quite possibly, although all they have to do to stop it happening is leave me alone.
¹ The only shop in the world where the packaging is more nutritious than the food therein.