Product: Credit card or prepaid card

My wife and I have made several purchases over the last year on Amazon utilizing their Equal Pay offer. This allows for a set amount of monthly payments without incurring interest. The Chase Amazon card statements provide an " Interest Saving Balance '' every month that is the minimum amount needed to pay that month 's share of the Equal Pay purchases, along with any regular one-time transactions for that month. For many months of statements, our Interest Saving Balance was between {$1000.00} and {$1500.00}. 

Strange activity began to occur with the XX/XX/XXXX statement/payment. On XX/XX/XXXX, I accessed the Chase mobile app to set up a payment for the Interest Saving Balance ( {$1400.00} ). I scheduled the payment for XX/XX/XXXX, the day before the due date. On the morning of XX/XX/XXXX, I saw that two payments had gone through. One for the amount I specified and one additional in the amount of {$1200.00}. I immediately called Chase to tell them about this overpayment. I was told that a payment had been set up through the mobile app on XXXX for the additional amount. I told them I did not do that and there must be some kind of glitch in their system. We didn't even receive a scheduled payment confirmation email ( as we did with the payment I scheduled on XX/XX/XXXX ). They agreed to refund the overpayment and I thought the matter was resolved. 

Fast forward to yesterday, I went to schedule a payment for the XXXX statement and found that the Interest Saving Balance was over {$4000.00} and that 4 out of 5 Equal Pay balances only had one payment left on them ( as of the XXXX statement, they all had 4-12 months of future payments on balance ). I then went to check the XXXX statement and found that on that statement, there was only 1 Equal Pay purchase listed. I called Chase this morning to flag the growing issue and was told the returns we had credited on the card were put towards the Equal Pay balances. We have had this card for many years now, and have never heard of anything like this. We checked the cardmember agreement and couldn't find it and the Chase representative we spoke to told us she would " have to get back to you because I can't find it right now, but I know that's how it's processed. '' This doesn't make any sense because we return things, particularly on Amazon, very often, and we have never seen this affect the Equal Pay balances. Furthermore, even if this was how the card worked, it doesn't make any sense to have Equal Pay balances disappear from one statement and then re-appear with a balance on the next statement.

My wife is about to go on maternity leave ( purchases for the incoming baby make up the bulk of the Equal Pay balances ) and my industry is currently experiencing a strike. We can not afford a $ XXXX payment right now. We asked for the due date to be extended or the interest to be waived until this could be sorted out and we were told that wouldn't be possible. So we're now facing down this very large, improperly accounted payment due in 6 days. 

I am also concerned more broadly that, based on what Chase representatives have said regarding this kind of balancing, this could be a more systemic issue affecting other consumers. It doesn't make sense that a sudden, new process of balancing, not specified in the card terms, is being explained by representatives as normal. Especially when they, themselves, can't direct me to the language in the contract that lays this out.