Airport security mailback?

I recently had a pocket knife confied from me at DIA and had to use one of those mailback envelopes to send it to my house from the airport. It's been almost a month now since I sent it and still no sign of my pocket knife. I HAVE been getting the shipping charges on my credit card, though. Who can I contact to clear this up? Someone told me that TSA pretty routinely steals stuff from those packages... :PP