bad service

I bought a phone for our 7 year old for Christmas, T-Mobile  took it upon themselves to add more data without asking at $15.00 extra a month, I caught it last month on our bill and asked them to refund me because I did not authorize it and she credited my account. Now,  it’s on the bill for this month and they refuse to credit the account, I told them it was cancelled and refunded last month, they still refused, I asked for the supervisor and he was the same way, he explained how if you don’t get extra it’s going to run slow, he’s 7 and is basically on WiFi but it still remains the fact that you can not just put an extra service on someone’s account because you think you can, I have been with T-Mobile for 19 years, who treats there costumers like that. So, some of our phones will be paid off and as they get down, we will start dropping phone I will go with a prepaid phone. It might sound crazy over $15.00 but it’s the principle of it.