Synchrony Bank Credit Card Complaint

Synchrony Bank Credit card department,

Payoff process Florida

I was contacting one of my credit card companies to make a payment. They go by - different names ; -, CARCARE ONE and SYNCBI. I was offered this card when I took my car to a mechanic. This was over a year ago. I have made all my payments and now would like to pay the rest of the balance off, or at least the majority of it since their interest rate is 29.99 %, after a six month grace period on interest for purchases. I called them to make the payment using a much lower rate credit card and they informed me, they allow payments from checking accounts only. I find it hard to believe that this is legal. Please advise.