Credit/Debit Card Selection
It wasn't clear to me whether I was selecting "debit card" or "credit card" when I was making payment. And honestly, I didn't even notice the selection until after I'd hit the "Process My Payment" button. It led to a moment of panic. I hope your form defaults to credit card. Or else, you could ask customers to make that selection BEFORE they enter their card information.
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Terence Way commented
Please do not default to the payment method to ANY of the selections. Force the user to choose an option before they can submit the payment please. Defaulting to DEBIT CARD should NOT be the norm. Otherwise, there will be a LOT of new users not even noticing that the selection is there until it's too late (if at all).
(I had this happen to me the first time I used your payment UI. I do not consider this my fault or the fault of the user.)