You'd be punishing the mail carrier more than the company that sent you the junk mail though, since they would have to lift that heavy thing, and they're jut the messenger.
its coming to Christmas, so tell people who hand deliver not to write on or seal the envelopes, they are usually a bit bigger than normal ones and they are free.
then put junk mail with any other rubbish in the envelope and post it back to them without a stamp
The prepaid postage only accounts for the weight allowance of an envelope.
then put junk mail with any other rubbish in the envelope and post it back to them without a stamp