Product: Credit card or prepaid card

bought two XXXX XXXX back in XX/XX/XXXX, on an agreed equal promotional balance monthly for 24 months at no interest. ( making the min payment {$76.00} ). I could access everything online as I had signed up for the online paperless statements, including the " promotional purchases '' section, which gave details of how many purchases ( of the 24 month plan was left, what the balance was, and what the charged apr was ( always 0 % ). The min was paid every month on time and the balance was being paid off until XXXX  had a promotion in XX/XX/XXXX to " sell back '' old XXXX ( giving a {$400.00} credit for each, XXXX total ). Once the new phones were bought XXXX sent a " send the old phones back '' package and each of the 2 old phones were sent back. At the time, it was said XXXX would give the two {$400.00} credits back within 6 weeks of getting the phone. This didnt happen, one {$400.00} credit was given XX/XX/XXXX and the other XX/XX/XXXX. As soon as the new phones were purchased, XXXX  store financing ( XXXX XXXX ) removed my XX/XX/XXXX equal promotional balance off of my statements and no longer gave me access to any information other than what my " recent history '' was listed online. At the time they also had taken my min payment from the advertised roughly {$85.00} for the new phones and added the previous {$76.00} to it to give a min payment due of XXXX. This resulted in me calling them in XX/XX/XXXXbecause I didnt understand why my min was so much considering the purchase of the two new phones ( just over 2k, divided by the 24 month equal payments was far under the XXXX I was being charged ). They " couldnt figure it out '' and after about an hour of trying to get in contact with a supervisor, they said they would have to get back to me and call me back ... never did. I wasn't surprised so I made what would have been my min payment of XXXX because the supervisor said she would adjust it and everything should be find the month after. Once the two {$400.00} credits were applied, synchrony  back dated them to XX/XX/XXXX ( for why i have no idea ) but when I talked to them the first time the supervisor said that XXXX credit was applied towards my oldest 24 month pay off of XXXX balance bringing the balance down to XXXX for the old phones. ( great ) and that my min payment would be the {$160.00} until the remaining {$180.00} was paid off since i was paying the combined 2 min payments on the two promotional purchases. So fast forward and my three months of payments, plus an adjustment from synchrony, and when I called today the old phone balance is at 300 something ( from what I was told ). I again expressed the fact I didnt have access to my statements ( supposed to have been fixed the first, second and third time I called. The one statement I have ( given access to by their IT ) was the one when I bought the two new phones and it doesnt show the promotional balance of either of the 24 month equal payment plans. and as of today when I called the most recent statement for XX/XX/XXXX doesnt have any listed promotional purchases. I then asked why the {$800.00} credit and the min payments ive made werent applied to the oldest promotion, and they said only a percentage of the payment goes to the oldest and that even if i made a payment of whatever the oldest balance was, that it wouldnt go entirely to the oldest one, that there is an algorithm that determines that. uh what?