Dish Network Charged for unreturned equipment that WAS returned Florida

Been a customer with Dish Network for years. Had an issue with my dish receiver last November and called customer service about it. Spent over an hour on the phone trying to explain to the tech support that my receiver had gone out and told them I alread went through all the steps to see if I could get the receiver to work but they insisted I go through them again. They finally determined that my receiver was no longer any good. Duh. Anyways they informed me they would be sending me a replacement receiver and that I needed to send back the bad receiver. I received my replacement receiver on November 20th and returned the bad one the exact same day. A couple of weeks later I receive an email that I received a charge for unreturned equipment. This is BS. I contacted customer service but they informed me they could not help me because there was nothing showing that I returned my equipment. This is a load of crap of how they treat loyal customers who have been with them for years. I am seriously considering cancelling my service and doing what everyone else is doing these days and getting a streaming service because I refuse to pay for charges on equipment that I KNOW I returned.