U-verse Non-returned Equipment Fees

I've recently experienced the worst customer service experience of my life.  Two months after I cancelled my U-verse service, I received a bill for $482 worth of non-returned equipment (2 cable boxes and 1 modem).  My local UPS store does not keep tracking numbers of AT&T equipment returns, but were nice enough to comb through their transaction records to find proof I returned the equipment.  I provided the exact day and approximate time of return and the UPS store actually found two records indicating that I returned the equipment.  I returned 5 pieces of equipment in total (4 cable boxes and 1 modem) and the store splits returns into two shipments if greater than 4 items are returned (hence the two transaction records).

The two transaction records do not contain tracking numbers, but do indicate that I shipped items to "AT&T U-verse" for a transaction amount of "1 cent" and contain my signature at the bottom.  AT&T will simply not accept this as proof that I returned the equipment.  I've escalated the issue numerous times.  Further, they can't even confirm that they received 2 pieces of equipment and that 3 remain missing.  It seems they never even had a record of me owning 5 pieces of equipment.  

Long story short, I'll never pay this bill.  I've never missed a payment my entire life and have near perfect credit.  I'll never do business with AT&T again.