I have great pleasure in receiving junk mail. Why? Let me tell you.
Some mail have BRE's (Business Reply Envelopes) and I like to take it and stick it back in the mail box. Sealed and empty. Companies do not pay for the postage when it is aent to you, but they do when you send back the BRE. This charge can be anywhere from 20 to 50 cents (USD). You can also get creative and stick the BRE on a box and fill the box up, costing the company much, much more money.
Have fun with this info.
@JGeorge Correct. My brother got away with taping a BRE to a box and filling the box with rocks. The company was charged charged close to 30 dollars just to ship it back to them.