You can upgrade the trial account in it's account management online.
The EU site to do that has been down for some time (at least two days). It gets all the way through to the final stage (after you enter the credit card details), then:
If you are attempting to purchase World or Warcraft online, please be advised that the online upgrade feature is unavailable while some improvements are being made to the system. However, accounts can still easily be upgraded to retail accounts by purchasing a boxed copy of World of Warcraft- available at retailers everywhere.

If you have already purchased a retail copy of World of Warcraft, you can enter your retail authentication key at the bottom of the Account Management home page.
I ordered 3 new boxes from Amazon, and they'll be here before midday tomorrow :thumbup: