Also, make sure that your credit card allows international and online payments. If it does and you still have trouble with our credit card payment system, please try using the PayPal interface.
Please try refreshing the website a few times and see if the version info at the bottom of the website changes to a higher version. Trying to make a payment with a previous version of the website might result in an error. Also if nothing else seems to change anything, please try erasing cache memory in your web browser settings and try again.