Product: Credit card or prepaid card

I am filing this report in hopes that you will be able to help me get this matter resolved once and for all. I have been trying to get this matter resolved for almost 8 months now to no avail. In XXXX purchased a pair of pants at JCPenney for {$48.00} with my credit card. When I received my monthly billing statement for XXXX I sent my payment for {$48.00} on XXXX. Synchrony Bank received my payment on XXXX. The due date on my statement was XXXX, thus they received my payment well before it was due. 
In XX/XX/XXXX, I received a letter dated XXXX ( letter # XXXX attached ) from Synchrony Bank advising me They had not received a payment for the past due amount ( {$48.00} ) on my credit card. I immediately called them and advised them that through my research I found that they had received by payment on XXXX and it was credited to the wrong account. Synchrony Bank checked into this matter and sent me a letter dated XX/XX/XXXX ( letter # XXXX attached ), advising me that they had indeed received my payment on XXXX and the payment was corrected where applicable the had adjusted the amount of any interest charges or fees that were assessed to my account as a result of the delay in posting my payment. At this point it appeared that everything was taken care of, so I thought, apparently I was wrong. 
I received another letter in XXXX dated XXXX ( letter # XXXX attached ). I called Synchrony Bank once again and finally on XXXX I was able to speak to someone at the bank. Per their letter they advised me that they had lowered my credit line to {$140.00} because of an external credit score they received from the Consumer reporting agency. The next thing I know I receive another letter from Synchrony ( letter # XXXX attached ) advising me I was required to make a payment on my account on the {$120.00} balance that was past due. 
At this point I was livid, I called Synchrony Bank again. As you can see they would never call me they would always send me a letter. They sent me another letter dated XXXX ( letter # XXXX attached ) advising me that they were unable to remove these fees and interest charges. They also stated that information about how the interest charges and late payment fees are assessed and how these fees may be avoided can be found in my card holder agreement as well as the payment information section on my statement. How can I be assessed late fees and interest each month when my payment of {$48.00} was sent them by my bank on XXXX and received by Synchrony Bank on XXXX well before the due date of XXXX. I should never have been assessed these charges and that is why I am asking Synchrony Bank to credit my account for all the late fees and interest charges they assessed on my account. I sent the payment in well before it was due.

To make matters worse I received another letter dated XXXX ( letter # XXXX attached ) which is identical to ( letter # XXXX attached ). They did change the date on the letter from XXXX to XXXX. Both letters # XXXX and # XXXX were of no use to me. Then they sent me still another letter dated XXXX ( letter # XXXX attached ) advising me that it may take them one or two billing cycles to look into my account and respond to me, Today is XXXX and I still have not heard anything from Synchrony Bank/ JCPenney Credit Card Services It is almost 3 months that I am wating for a reply from them. I need to get this matter resolved as soon as possible. I dont want my credit score to be hurt in anyway by this matter. I suspect that may already have been. Thank you for any help you may be able to give me.