Xfinity flex streaming box

I upgraded/changed my plan as my other 12mos contract ended. Received the new Xfinity modem and took my other to return at an Xfinity store. When I returned my old modem, I also returned the streaming box not realizing that this was supposed to be maintained for my current plan. I checked with the rep first, he assured I should return both items. My husband and I left the store and I still felt uneasy, so we went back, referenced that the receipt only listed one cpc code. Rep again assured returning both was appropriate. I'm seeing the flex box is still listed on my current plan meaning there would be an expectation it's returned at some point but I no longer have it, it's with Xfinity and I was assured by a rep this was correct. I still have my receipt and can reference the times I was there/returned. How do I address this so I'm not inappropriately charged at the end of this contract for not returning equipment that's already been returned?