Product: Credit card or prepaid card

I opened up a Amazon Store card with Amazon Prime " Synchrony Bank '' in XXXX. I started using their promotion for amounts of {$140.00} or more that can be paid 6mos. later with deferred interest rates. Interest is not supposed to be charged if paid by the 6mos. due date. 
I opted to do this and have made payments over the phone or online by asking to have my payments specially allocated for the " promotional purchases '' that will be expiring on the day of the payment was made or before it was due. 
I made sure that before making the payment that it would be allocated properly to the expiring promotional purchases. I have had issues with paying online because a month later when I call, I look at my statements and saw that I was charged interest for a promotional purchase because it was not paid off. I was then told that I should call to allocate those payments over the phone. That is what I started doing. I called each month to allocate and pay off the promotional purchases specifically. 

However, I continued to have issues even though I had an associate help me allocate payments over the phone. The issue turned out to be that " somehow '' those payments were not paid off and I was charged interest. The previous associate had made an " error '' or something. 
Sometimes I would call them to make sure that I didn't have any interest charged for just the expired promotional purchases. I know that I am charged little interest for my non-promotional purchases ( which is just like any other credit card ). 
I would be assured by the associates that there was no issue. Usually the associates were from XXXX noticeable through their accents. It is only up until recently I noticed that Americans have started to answer the phone. 
Couple of months ago when an American answered the phone and I asked have I been charged any interest on the expired promotional purchases in the past according to my account. I was told yes. I went ballistic because I was just asking to make sure and expected them to say no as the previous associates assured me. Many times my statement of whether the promotional purchases have been paid off and what the associates see from their system is not in line. They say something else so it makes me think that there is something fishy going on. 

When I was told a couple of months ago that I was in fact charged interests on an couple expired promotional purchases, I told them that they had to go back from the inception of my account and when I first started using their promotion of pay 6mos. later for purchases made $ XXXX with deferred interest if paid in full by the dude dates. 

They had only gone far back to XX/XX/XXXX - XX/XX/XXXX. They reversed nearly {$300.00} in interest rates that I should not have been charged. This issue went to their corporate and I was told that they had no information whatsoever about any payments I made toward my promotional purchases prior to XX/XX/XXXX. I said that that did not make any sense! How could they not have a record? 
The associate said that they could not go back further to look and that they had no information on my account previous to XX/XX/XXXX. Worse is the fact that this was the second time I had asked their accounting team to go through my account to check for any charged interest on promotional purchases. They reversed some interest and told me they had thoroughly checked my account. But months later in XX/XX/XXXX when I called to ask if I was charged interest on my account for expired promotional purchases, they said that on such and such months I was charged interest for expiring promotional purchases. 

The whole duration of my having this store card, I had paid off in full ALL of the promotional purchases I have made in the past without accruing interest. That was the whole point of my using their promotion so that I didn't have to pay interest for XXXX. 
I asked them for a copy of my credit statements from XXXX of when I opened my card and up until now. They have access to that but not my " account '' information through which they can tell me whether I was charged interest for any expired promotional purchases. I have made XXXX purchases using their promotion offer. I have had many many arguments over the phone with them because my payments toward promotions did not line up the next month when I call to make sure it was applied correctly. 
I am certain that I have more interest charged from past expired promotional purchases that they do not want to disclose or reverse, which is why they put a stop to it because they know they owe me money. 

I am not happy with the fact that I was constantly charged interest on promotions that I paid off. I do not trust this company and I believe that they are refusing to go through my account from XXXX until XX/XX/XXXX because they practice fraudulent activities with their promotions to charge customers more interest. The company should have a record of how many times I have called to make purchases and notes. 
Please help.