Mail carriers are not idiots, they just discard the thing as trash since its obviously not an envelope… Business reply mail will only ever be paper letters, so what you actually want to do is just send whatever it is back to them with useless information or if its an actual envelope you can stuff it with glitter or shred the paper inside, its still technically “improper use” but they won’t know that until its at the destination.

Note to anyone playing at home: if you send any trash, make sure it doesn’t include anything with personally identifying information on it.