“someone told me that you can tape the return envelope on a box and seal it and send it.That way they could get a big bill.”
It was always my understanding you could legally only put in it whatever they send you. IOW, it would be a no-no to attach a business reply envelope to a brick. I do however, include the original envelope and make sure it is too lumpy to go through the sorter. Sorting by hand does cost them a bit more.
I’ve mailed several concrete blocks with no repercussions...