The usability of the payments form is poor. It doesn't state which fields are required (if it's all of them, it should say so at the top). I understand not remembering the credit card number or CVN between requests, but it also forgets the expiry date, and the country. I had to resubmit several times because of this. Fortunately in my case I had the patience to go through it regardless, but given the amount of research indicating people's willingness to drop out of payment processes halfway through, this is something really worth fixing.