Do you get stuff in the mail that you never asked for? Like those credit card applications with terms that are just plain awesome! It is a real shame to go to all the trouble shredding and disposing those when you could just put the postal service back to work on the sender's dime.
I learned this technique from a lovable grandmother. She was a kind and gentle lady and I'd never cross her.
Take all the material that was in the original envelope and put it into the reply mail envelope. Put back into the mail.
I can't see anything wrong with this as you are not writing messages or putting garbage in the reply mail. You are just giving it all back.