Still Being Billed after Equipment Returned

I returned my Directv equipment as requested when I cancelled my account back in September and have an email receipt that the equipment was received but am still being billed over $145.00 from Directv saying the equipment wasn't returned.  After 2 phone calls to customer service, this is still not resolved! Each customer service rep has stated this has been escalated and not to worry about it but I just got another reminder email that my account is past due. Very frustrating after being a loyal customer for over 15 years.