Charged for a third box we didn't have for 31 months

I have spent hours trying to resolve a billing error. Everyone-xfinity staff agrees, we were overcharged. The last specialist said she would get back to us and we have not heard from them. This is the third agent to not follow through. We were charged $10.00 a month for 31 months for a box we did not have. I noticed the error and contacted Xfinity last November 2023. I was given some  -$200 credit (should be $310) and then it was removed and we were charged! ? Not sure why. Now I keep getting billed and late charges for this issue. I continue to pay my bill as I always have but I should not have to pay for the overcharge/mistake Xfinity made. You erred, not us. How is this ethical? Please credit me and remove late fees as well. I really want to resolve this issue.  Please credit us for the mistake Xfinity made. I want to keep our service with you but am feeling so frustrated with this process. Ready to look for a new provider....