I haven't figured this part out yet. I've been billed for 10 domains but they are all listed as 'payment pending'. It's completely crazy. If you figure any of this out please post here, I promise to do the same.
The credit card I have on file is old so the payment didn't process, but it created an invoice. I was then able to go to 'Payment Options' in the sidebar and click 'PayPal' and then pay the invoice. That renewed the domain instantly (or actually, within about 10 minutes after I received the emails).
Be sure to set all of the domains you don't wish to auto renew to 'delete at expiration' or they'll auto bill your credit card on file. They reset them all to auto renew by default.
