One nice UX feature that I like on this site was the way in which they highlight where to find your credit card CVV/security number depending on which credit card you are using:

Visa card with CVV number highlighted

The image above shows the security number highlighted on the signature strip when you select Visa from the credit card type drop-down list. Compare that with the screenshot below showing the security number highlighted on the front of the card when American Express is selected:

AMEX card with the CVV number highlighted

It’s a simple feature really, but nicely implemented.