tmobile kept billing after I canceled

I had 3 lines with tmobile. One belonged to my ex and I wasn't going to keep his line any longer. I switched two lines to verizon and called tmobile on June 11th to cancel my entire account. They told me all was done and the line I did not transfer would be no longer in use after June 26th....the end of the billing cycle. I get the auto payment taken out in July for the billing cycle ending June 26th. Then August comes and I get an automatic payment out for July. I called tmobile and was angry. The guy was very nice and could see in the notes I had canceled on June 11th and that they would be refunding the money back to my bank account. A few days later the money was put back. Fast forward to today....in the mail I received a bill from t mobile from what they had refunded me with a late fee added. I am livid, I call and get nowhere so I ask for a supervisor. She tells me it's too bad cause my exes line wasn't canceled until August 26th so it's a legitimate charge. I spent 90 minutes going round and round and even she can see the notes that I canceled in June but says that the line got used after June 26th so I have to pay. I told her if it had been canceled there's no way my ex could've used the line so it's on tmobile. I got nowhere. This is complete bull and I feel very unethical.