Product: Credit card or prepaid card

Good day, On XX/XX/XXXX I did a XXXX XXXX Promo Purchase and by XX/XX/XXXX I paid off the full balance, so I wouldn't have to pay any promo finance charge. 
On XX/XX/XXXX I did other XXXX XXXX Promo Purchase for {$5600.00}. During the rest of the year, I was paying the minimum due. On XX/XX/XXXX I was doing regular minimum due payments. The promotion was about to expire on XX/XX/XXXX, so on XX/XX/XXXX, I made a payment of {$2400.00} using the first bank account on my portal, I was rushing to pick up my toddler and I didn't double check the account that I was using to make that payment. 
Until two days later I realized that I was missing {$2.00}, so I immediately paid it using the second bank account ( which I thought I used it in the first place to pay the {$2400.00} ). 
On XX/XX/XXXX, I got promo finance charged {$1200.00}, but I found out by an email until XX/XX/XXXX. I immediately call the number XXXX, I spoke with a representative and I explained that it was an honest mistake, since I already have a record that I paid on time the first promo purchase, I asked her to allow me to pay right away the {$2400.00} so the promo finance charge could be removed from my account, but she told me that she couldn't help me. I requested to speak with her supervisor and I talked with XXXX ( he didn't tell me his last name ) I explained everything again and I asked him several times to work with me, so I could pay immediately the amount owed. He kept saying that he was not able to help me in any way. 
I called again yesterday, I spoke with other supervisor named XXXX and I told him that they never notified me ahead of time, there were 3 days before the payment was due from the time that I paid and I didn't get a notification about it. Again, he said that he was not able to help me. 
The fact that this Bank didn't notify me, implies an intent to deceive the consumer, there was no transparency on their part since they were aware that the payment got rejected and therefore, they took advantage of an American consumer, which is me. Because my payment was done on a business day, they would have been able to notify me before XX/XX/XXXX. But instead, I got an email until XX/XX/XXXX saying that I owed them {$3700.00}. If there was not a deceptive financial practice in their part, why they didn't notify me during the small window to fix an honest mistake? This actually demonstrates bad faith on their part, because by not letting me know, they harmed me, financially speaking. 
If I wouldn't have the intention to pay, I would understand their position by not removing the promo finance charge. However, I submitted the payment, I did it 3 days before the due date and I got 2 emails, the first one on Friday XX/XX/XXXX and the second one on Monday XX/XX/XXXX, that said thank you for your payment. This is confirmation that your payment toward your XXXX XXXX  credit card has been processed. Thank you for your timely payment. Based on those emails, I thought that I had paid off the full balance as I did in fact with the first promo purchase Please, I need you guys to help me to make this company drop the promo finance charge, it is unfair to charge me {$1200.00} since there is evidence that I made the payment of the full balance 3 days before the due date. Even though it didn't go through, they could have let me know by an email that the payment was returned and I would have been able to fix the mistake immediately. Instead, they sent me an email confirming the payments and, 20 business days later I got other email indicating that there is a promo finance charge. 
Based on the evidence that I paid off in time the first promo purchase with them, they should give me an opportunity to pay the {$2400.00}. I'm not asking this bank to waive {$2400.00}, I'm just asking not to pursue an unfair enrichment and unjust practice by making an honest consumer pay an extra {$1200.00}. 


Thank you for your time.