ATT still billing me for returned receiver; The UPS Store gave me a return receipt but Customer Service says ATT has no record

Hello! I returned my second U-verse receiver via The UPS Store months ago but am still being billed $10 per month. AT&T Customer Service has told me twice it's not not their problem. The UPS Store says the "AT&T Equipment Return Receipt" it gave me is proof I delivered the equipment as instructed by AT&T, and that AT&T's issue is with UPS and not me. How can I (1) submit my return receipt to AT&T; (2) stop being billed for the receiver, and (3) get a refund or credit for the incorrect billing? Thanks!