Product: Debt collection

I made a purchase at XXXX on XX/XX/2021. The sales helped me apply for financing for the purchase, however they told me I need to pay off the purchase by promotion period I told them ok, I'll just set up direct deposit, they take what they need within that promotion period. 
The items were sent to me XXXX damaged. XXXX was really helpful and placed a replacement order for me. After getting the furniture, I waited for months to get my statement. Right after I got my first bill, I saw that it was late. So I called immediately to ask that they bill my account for the needed payments ( I forgot when that call was made ). 
Next month, I see the funds drawn out of my account. And it has been drawn out of my account since. I thought all was good, and everything will be paid off in the agreed time ( as all my other finances ). Until I got the last bill, this is a bill for XXXX that I just got. I saw my total was at XXXX which was strange because my purchase was for XXXX. When I saw that, I immediately contacted the card issuer ( synchrony ) and they told me it was $ XXXX interest. I question the rep and she told me she can do nothing about that. I called the store, thinking it was store problem when they told me this is the bank ( synchrony ) issue. I call synchrony again, asked for a XXXX, and the XXXX basically told me, doesn't matter I had funds on my bank account, their system indicate I chose to pay only minimum ( false because I told the rep who took my first payment, that all that was necessary to get this paid off in a timely manner ). I tried to tell her her reps set up my account wrong but she refuse to listen. She said the minimum collection was written on the bills, why did I not pay attention, find the error, and corrected them? Why did I not pay attention to how they collect my money? 
I'm practically speechless to that. I gave them full access to withdraw from my account, I keep watch to make sure there was at least {$2000.00} in the account so their payment will not be blocked, and I still need to make sure their staff wrote in the right # on they system so they could collect the right payment from my account? Ok, even if this is my responsibility, to make sure she's doing her job right. Would it be too much to ask that she notify me that she's about to charge me $ XXXX interest? I finally saw during my call with the synchrony XXXX, that if I was clueless to her methods of collection, I will have to pay $ XXXX for a {$4000.00} debt, and that is 17 years of payment plan she had for my account. 
I can pay off whenever but I refuse to let synchrony get away again with cheating people out of their hard earned money ( i have heard others complain that they do this to all their customers ). I checked with other lenders ( not synchrony, another bank institution I have finance with ), they all say they have a rule of at least attempting to call their customers before the promotion period for debts ended because the interest would be costly for the customer. I got nothing, no call, no mail notice -- no i got a mail, a super late statement ( issued XXXX, I received XXXX ). I want them to confirm their rep had made a mistake and that they will fix their issue. When they fix, they can contact me to have the account paid off and closed.