Directv I'm being charged for equipment I returned. Internet

After suffering through our entire two year agreement with Directv (after being promised a low price and all the local channels and then being charged more than twice as much and losing many of our favorite channels including local channels), we were finally able to disconnect.  We received the boxes to return the receivers about a week after cancellation and we immediately packaged up the receivers and dropped them off at our local shipper the next day.

Well three weeks later, Directv is billing us for an equipment fee and stating we did not return the equipment.  I tried calling customer service - got some guy that I could not even understand who was not willing to help me.  I also requested help on their website and I have received several emails "apologizing" but stating they can't find the receivers, so I owe the fees.  (Thanks for all your HELP).

Now why would I want to keep your receivers?  I don't even want your service.  I have witnesses that can attest to both seeing me package up the receivers and drop them off with their mailing labels.  I have the "tracking" number (however it comes up as invalid on the FedEx site - so I'm guessing Directv gave me a bogus #).  

I'm certain the receivers are sitting in their warehouse in some stack but in the meantime I'm stuck having to jump through hoops and fight with Directv over this $326 charge.  Does anyone have any advice on how to get directv to back off and find the receivers on their own end before harassing me?