I like the basic, easy design. One suggestion though (take this with a grain of salt though, since I am a backend programmer and not a design specialist). I didn't see the 20 euro preauthorization message the first time the new card dialog came up, and instead I closed it and went to the billing faq to find out what that was about when I saw Visa asking me to approve a 20 euro charge. I saw the message the second time around, but perhaps making that message a little more prominent would be helpful, especially for simpletons like me to tend to miss things.