Product: Credit card or prepaid card

In the first week of XX/XX/XXXX, I made 2 payments to my account to resolve a balance that had recently changed from promotional to revolving. According to my XXXX statement due XX/XX/XXXX, the total balance of {$5300.00} was owed and {$4300.00} of that was on a promotional 0 % APR through XX/XX/XXXX. This meant that {$1000.00} was revolving and incurring interest at that time. I made a {$180.00} payment on XX/XX/XXXX and a {$880.00} payment on XX/XX/XXXX totaling to {$1000.00}. This should have resolved the outstanding revolving balance and I would have expected some very minor remaining interest for the few days past the statements due date that the 2nd payment was made. However when I checked the account online later, it still showed the revolving balance outstanding along. I spoke with a woman in customer service who said that I was responsible for calling into them to ensure that my entire payment got applied to the revolving balance. She was going to waive the interest and process the application of the payments as discussed to the revolving balance so it would be {$0.00}. I do recall asking if I need to make additional payments and was told no. I assumed it was all fine until I went back to the account to pull records in XX/XX/XXXX for accounting/tax purposes and surprised to find that not only have I been charged interest monthly since XX/XX/XXXX but also that a portion of my promotional balance good through XX/XX/XXXX had been changed over to revolving and compounding interest on that balance. Statement and transaction history showed that a {$910.00} promotional plan transfer was done by the representative in XXXX. I called customer service back on XX/XX/XXXX and manager XXXX pulled the call recordings at my request. He listened to them, understood the problem, and said he was taking the actions to correct the prior representative who did not process it correctly. The penalties would be waived and the balance remaining would be corrected back to promotional only. Due to that day being the end of the billing period, he recommended to check back in 2 weeks to ensure the changes took effect and follow up on further interest fees to be waived. Shortly after I received a letter in the mail confirming that a complaint was opened for a special department to review. Unfortunately, I was ill and hospitalized for some days. I was only able to check back on my online account XX/XX/XXXX to find a credit of {$140.00} applied and my revolving balance was still present and increasing. I called that day to a supervisor XXXX who advised he could not do anything due to a new letter issued closing my complaint. He said the letter confirmed that there were no issues and although he could not see the letter, it supposedly gave explanation and the amounts I currently owe. At the time of the call I had not yet read that letter and have done so now. The letter did not state what I owed nor acknowledged or explained why a promotional balance portion got moved to a revolving interest-incurring line. The letter stated for further inquiries to be made through the Customer Care line. I called again and spoke to manager XXXX ( not the same prior manager XXXX I had spoken to ). I have explained this several times and he advised that he can not do anything further as the complaint had closed out and is not a correction he could make himself regardless if he understood the issue. I could only request a second review be done by submitting so in writing.