Still billed after cancelling service

As many others on this forum, I have cancelled my plan and was still billed. It's been a frustrating and confusing experience, to say the least.

I signed up for a 1-year contract, but there have been some career changes so I'm moving earlier than the contract's end date. I made an in-person appointment at my local Xfinity store and cancelled and returned my equipment on 12/28/2023. I know that I cancelled in the middle of my cycle– My cycle is XX/21/XXXX to XX/20/XXXX.

Xfinity bills in advance and my payments usually land on the 12th of each month. So, my bill on 12/12/2023 was for the cycle of 12/21/2023 to 01/20/2024. The customer service representative I talked to at the store said that I would be charged a $10 early termination fee (ETF). I'm in agreement with all this.

I talked to a live agent on 12/31/2023 to confirm what my final bill would look like, and they said that I would be charged a total of $18.17. $10 for the ETF and $8.17 for the usage of 12/21/2023 to 12/28/2023. Didn't my bill on 12/12/2023 already cover the usage from 12/21/2023 to 12/28/2023?

Fast forward to today 01/12/2024, I was charged my regular bill. Why am I still getting charged? If Xfinity bills in advance, then it means I've just paid for the cycle of 01/21/2024 to 02/20/2024, even though I cancelled my plan on 12/28/2023.

I talked to another live agent this morning, and my questions weren't resolved. They confirmed that I should be receiving a credit of $18.06 for the unused internet plan, but it still doesn't make sense WHY I'm being charged this month when Xfinity bills in advance. Shouldn't this mean that I should receive a credit of $54.29? $18.06 (unused plan from 12/21/2023 to 01/20/2024) + $36.23 (wrongfully charged to me today) = $54.29.

To make matters worse, I can't delete my Payment Method from my account. When I click into it, it says that it's been restricted. I want to resolve this matter before going to the bank to make a dispute.