Some folks have been having problems with paying online without a PayPal account.
Apparently, some folks don't notice the little line down at the bottom of the PayPal dialogue that says:
Don't have a PayPal account?
Use your credit card or bank account (where available). Continue
Please notice in the picture below that you can click on the "Continue" in order to have PayPal process your payment information without having an account with them.
Have you already registered, but you now want to go back and pay? Do you know how much you need to pay? Try this direct link "iptc-PAY" button in order to send the SCS your payment via PayPal. Just enter the amount you owe, and proceed on.