How to ask Xfinity refund money when charging customers extra money

I recently close my phone plan with Xfinity but they still charged me one more billing cycle. I contacted the customer service for refunding 4 times, 4 times! And every time the agent admitted it is a billing mistake and promised they would refund within 3 days, but never solved for lasting longer than 1 month. Then I called the credit card service to help solve it, and just 1 day the money got back. I could not trust Xfinity since this and then begun to check the billing history and found Xfinity even did not give me devise discount when I enrolled. That was about 3 years ago quite busy period for me that I thought the promotion happened already and I did not check the payment for it. So I contact customer service for this and wonderful they performed again the way of talking a lot plus no solution. Anyway to solve this?