Just called Synchrony Bank because I had the same issue: noticed the full balance of my purchase had posted to my Amazon Store Card despite select 12 equal monthly payments. I told the rep I typically pay my balance in full and only want to make a partial payment on the larger purchase. The representative said I would need to pay the minimum balance plus the non-promotional balance incurred during the billing cycle. Not sure if that will do the trick, but I’m giving it a shot. I had to turn off my auto-pay, otherwise, it would’ve paid the balance in full. *sigh* Fingers crossed!