Years ago you could simply tape the prepaid business envelope to a 2”x4” block of wood, a brick, or anything else heavy and the mailman would take it. Too much fun...
Yeah, postage on reply envelopes used to be unlimited in the US. In grad school, there were lots of lead bricks around. So we did that once or twice, for laughs. But you know, the lead was actually worth something. But not as much as the postage.
That stopped working in maybe the 80s.