Victor J. Osborne, Jr. wrote:

Do what I do and send the stuff back. Use their envelope and put it all
back in and send it.

A totally useless gesture. There's this myth that it will cost them
money or teach them a lesson. All it will do is annoy the minimum wage
employees in the mail processing company that the sender hired to
process their mail. The return postage for people who do this is such a
drop in the bucket that it doesn't even show up as a line item on the
bill that the processing house sends to the organization that hired them.