Product: Credit card or prepaid card

I am signed up for paper bills. I did not receive my XXXX Statement ( XX/XX/XXXX - XXXX/XXXX/XXXX ). I did not realize it until I went to pay my XXXX statement and saw that there was both an interest charge and past due balance. I pay the balance due each month so as not to have to pay interest. On XX/XX/XXXX I called Barclays XXXX XXXX XXXX XXXX and explained the situation. They reviewed my history and agreed to reverse both the interest and late fee. Although my payment was not due until XX/XX/XXXX, I asked the agent if it would be better to pay over the phone to make sure everything was resolved so I would be back to a " XXXX '' balance. She agreed and took the total balance due ( {$2100.00} ) subtracted the {$40.00} interest and {$29.00} late fee and said I owed {$2000.00} and I gave her my checking account information to pay immediately. 
My next statement, for XXXX ( XX/XX/XXXX - XX/XX/XXXX ) again showed an interest charge of {$30.00}, and it did show the credits for the previous interest and late fees. I called again on XX/XX/XXXX to get an explanation. The agent said it was an error and said she would reverse the fee as I should not have been charged the interest. Again, I specifically asked her what I needed to pay. The agent subtracted the {$30.00} from the statement balance of {$440.00} = {$410.00} This time, I chose to pay online so I had a clear record of payment on the website. On XX/XX/XXXX I paid {$410.00}. and assumed that NOW it would finally be all resolved. 
XXXX Staement arrives and AGAIN there are more interest charges for {$39.00} AND I was never given the credit the agent promised for the {$30.00}. I called again on XX/XX/XXXX and was on the phone for almost 1 1/2 hours. The first agent said that she " was not authorized to remove interest charges that high '' which made no sense since the previous agent on XXXX was able to remove over {$40.00} in interest. She transfered me to a manager named XXXX. She said that she would be able to remove the interest charges but that I had to pay the full balance due ( including BOTH the {$30.00} from XXXX and the additional charges of {$39.00} for a total of {$70.00}. XXXX. She said that " the system '' would take 2 days to issue the credit for the interest charges and since my bill was due the next day, XX/XX/XXXX, I had to pay the full amount and get the credit in the next statement. I was very upset, and XXXX continued to interrupt me and failed to understand that I did not think it was right that I would have to put out over {$70.00} now for fees that several agents had now agreed I did not deserve to be charged. Recall that part of those fees were because the agent in XXXX did not do what she said she would, so the charges were not reversed. In effect, Barclays is holding over {$70.00} of my money for a month before I actually see the credit. She continued to argue with me that I would get it back in a few days. I told her that a credit towards next month 's bill is not the same. I was paying in cash, the $ was leaving my account that day. Eventually she rudely disconnected the call without telling me, I was put in a que and XXXX, another XXXX answered the call. I went through the entire story all over again with him, my anger intensified because I had been hung up on so rudely. XXXX explained that the credits for the interest take 2 days to get issued and re-stated that I had to pay the full amount including the interest. He also informed me that there would be ADDITIONAL interest charges that would hit my account on XX/XX/XXXX even if I paid the entire amount ( including the interest charges which everyone agreed I should not be charged ) on XXXX XXXXXXXX. My understanding is that because the agent in XXXX did not do his / her job and did not reverse the charges as promised, my account is now flagged as not paying the full amount due for 2 consecutive months! I am a good customer and always pay the full amount, I never carry a balance and never have in all the years I had this account ( 5 years??? ). I was flagged due to their agent 's errors and misinformation!! He agreed but said that there was nothing he could do, that I would have to pay the amount including the unjustified charges, and call back again next month and go through this all over again! There does not appear to be an end to this! They are going to keep on charging me interest, forcing me to pay, and then MAYBE issue a refund if the agent decides to do their job. Meanwhile, they are taking my money and using it for the month and probably making money on my money!!! This is a vicious cycle happening to a good customer who has done nothing wrong and has excellent credit!! If I had truly been acccurately informed, I would have just stopped using the card until it was all fixed, but it was only the 4th agent, XXXX, that truly explained what was happening ( even though he could not/ would not fix it ). There must be a law protecting customers from this kind of activity!! Please help me!!