Xfinity charging me for "unreturned equipment" that I did, in fact, return. Help!

I returned my Comcast equipment to a UPS store after the Xfinity store in my new community told me they were not able to process returned equipments from other regions. I returned all three pieces of Comcast equipment to a UPS store, simple as that, they told me I was all set! Fast forward three months later, and I am receiving threatening emails and texts from Xfinity, saying that they will send me to a collections agency if I don't pay for the equipment they say that they never received. I am being held responsible for equipment that I returned well within the return window because someone in the UPS > Xfinity workflow lost my equipment.

I have called Xfinity and explained this situation 5 (yes, FIVE) times now to various support reps. Each rep has told me that this happens, pieces of equipment get lost in the shuffle, and that they will update my account to mark the equipment as returned and remove the charges. But here we are once again, after phone call #5, and I am facing a collections agency because of someone else's error in the return process. I am a continued Xfinity customer at my new location, but have been really disheartened by the support that I have received, especially the threatening emails and boldfaced lies from Xfinity employees. It feels as though my hands are tied at this point and there is no one able to help me. Xfinity has backed me into a corner where I have no choice but to pay a fee (because they lost or improperly returned) my equipment, or face legal action. I am so upset with this lack of customer support and feel as though I need to close all my Xfinity accounts lest this happen again next time I move. Any advice is appreciated!