Allow prepaying or authenticated payment subscription for EU users

In the EU it's now required that credit card transactions must be authenticated on purchase or subscription. This means there is a risk of payments being rejected for EU customers as the only option when paying outside of the EU is to set a time limited window on when purchases from outside the EU can be made.

Having the option of paying "in advance" for the next subscription period would allow users to set that window, make the payment and then close the time window.

The best option would of course be if OpenDNS could enable user authentication for EU users, that would also allow OpenDNS to use the card information for renewing the subscription the normal way.