Those are typically paid by weight. Several spammers have gotten envelopes full of gravel because of me.

It's paid by weight and thickness so fill it with trash like pieces of cardboard and sand.

Or bricks.

Fuck, I'm gonna start doing this. I've tried to get the postal service to return to sender, but they never do.

I recently used such an envelope to return a sheet where I had just written "poop", using my left hand for extra aesthetics. I also added some screws and nuts I had laying around in case it'd increase the postage. My GF called me childish. I call it the result of ignoring the no-spam-registry everyone in my country is legally bound to follow.