I receive lots of junk mail and am not fond of the senders. It it legal to send them non-threatening objects (such as rocks with smiley faces panted on them) in the pre-paid postage envelope?
Cecil’s article on the subject:
Can I mail a brick back to a junk-mail firm using the business reply envelope?
It’s not against postal regs to mail anything that complies with postal regs.
So if it can’t be seen as threatening, I don’t see how anybody could complain.
however, that sort of mail is processed by machines. The mail is moved by heavy rollers, and rocks will rip and shred the envelope.
The intended recipient will not receive their rock and will not have to pay postage for the rock.
Quick answer is that sending stuff like that is just discarded at the local post office. So it doesn’t cost the junk mailer anything – it just costs us money by wasting the time of the locql postal workers.
And that article by Cecil is a bit dated by this: the standard 20-cent rate!
Its been a long tome since q stamp was 20¢.
Mass mailers and their spam are what keep the Post Office in business and their rates relatively cheap. Just throw the unwanted stuff away. It’s the least you can do to ensure the continued existance of the only self sustaining government agency. Just my opinion, anyway.
Stuff the return envelopes with pages from junk mail catalogs up to a half inch thick. They will make it through the system, costing the junk mailer about two bucks a pop. That way the P.O. makes some real money.
An old acquaintance of mine used to do this. It was a time consuming hobby of his. No idea if he still does. Lost track of him long ago.
Technically, the mailer can ask for a refund from the post office for any such misused business reply. Which would cost us postal customers money to process those refunds. Usually, the mass mailers don’t bother, but if it got to be a problem, they might.
Looks like it worked, (if you are a junk mailer).
I do this too. 
I also send back those postage-paid postcards. It takes, oh, maybe 5 seconds a day.
I usually tear up everything that was in their envelope to me, and send it back. I figure that costs them some time and money, and it lets me vent.