Synchrony Bank Online payment system seems purposely set up to confuse you when doing auto-pay Draper, Utah

In short, purchase made through Sleepy's, with Synchrony Bank handing account!  Five year auto pay, to be finished August, 2018.  All payments made, on time, however, if you set up payments on line, the system will only take one payment at a time!  Even though it gives you a space to add extra to your payment, the system will not recognize it, therefore it is not counted.  Since my account was to be paid in full by August, 2018 I set up the balance to be paid July, 2018, with an extra payment.  Two calls to bank this morning, to find out why my extra payment was NOT paid, and this is what they told me!  I asked why your given space to add extra to your payment as it doesn't make any sense;   and they could only say because that is how they do it.  The system only recognizes one payment at a time.  Which brings me back to when I first opened my account at Sleepy's! 

After making payments for several months,  (now, remember this was a contract for five years and if you miss a payment, you owe for interest!) they sent me an email stating that siince my account was on time they were rewarding me with a free month, of no payment.  I had figured in my "promotional payment" as well into the regular payment, so if I had not gone into my account and did some changing to add another payment for that month to pay the extra "promotional payment", it would have violated my contract and they could have charged me the interest for five years!  I think this payment system they have, should be investigated by the banking commission as it could be confusing to people and cause them to pay extra interest because they might think that they made an extra payment, and in reality, the system doesn't recognize it and I believe that it is probably set up that way, just for that reason.