If you receive pre-postage-paid junk mail, you can drop it in the outgoing mail, and mail it right back to yourself. Why would you do this? Well if enough people did this, we could rack up very large postage bills for the lousy junk mailers, and simultaneously help subsidize the post office with additional volume.
Besides, dropping something into the outgoing mail is just as easy as dragging into the house to recycle it.
Who wants to start a movement? Re-Mailers of America.