.............. AND I look for other stuff to send back with it, just to make it heavier so it will cost them more. 
Good man. Superior approach in thinking and execution.
About 10 years ago I flew next to a USPS Postmaster who lived outside of Tulsa. At that time, those little envelopes, when returned, cost the CC companies about $1.35 in postage fees each. 10 years later it's gotta be a lot more plus when you add their time, staffing and overhead easily $2.00 in cost for each returned envelope...I would suspect true cost in the range of $2.00 to $3.50 per envelope. Adding weight adds insult to injury...way to go.
FYI...some have wised up and put "window"s cut in the envelope so you have to return at least part of the offer in one piece....not a problem...can do. Anything to oblige in costing them time and money.