When you sign up to u2.com, it initially creates a free account, with limited rights (most noticeable in the Zootopia section, where you’ll only see one group). This happens before you even send them your credit card details. If for some reason you credit card payment doesn’t work out for you, you can try again later simply by closing your browser and logging in with the (free) account details sent to you. If you then click “subscribe” you’ll be sent on to the shopping cart script. Check the item “Membership” is only in there once before giving it your credit card details. The system should mail you again after your credit card payment went through.
A way of checking whether your payment went through is checking the Zootopia forum. If you’re a paid user, you’ll see additional forums in a “Subscriber Site” section. Also, under “My Profile” something like “Your Membership will expire on dd.mm.yyyy” will appear (that’s euro date format by the way).