Annoyed with Amazon Chase card

Last November I bought some furniture on Amazon and paid using the Amazon Equal Pay program which works out to $164/mth, no interest. I pay the rest of my balance off every month plus the $164 (the “interest saving balance”). My parents use my Prime account to buy stuff on Amazon and pay me what they spend. Usually I just keep the money they give me in my checking account until the bill is due, but last month my mom and I split a large purchase for my daughter’s birthday, and I didn’t like seeing my balance so high so I applied the payment she gave me to my account. Well, I just got my statement, and for some reason Amazon applied it to my Equal Pay balance. So now my interest saving balance due is $1200 more than I was expecting. I have the money in savings but it’s really going to make a dent. Honestly Amazon’s equal pay program has been really confusing to use, so I’m kind of glad to be done with it but I still don’t understand why they applied my payment to that program. Anyone have similar experiences with Amazon?