The following is taken from our Billing Support pages, Tazina, and may prove illustrative in explaining the proper course of action:
Q u o t e:
Subscriptions do require that some form of payment information (such as a credit card) be entered before activation; at this time there is no way around this. This is required in case account play extends over 30 days, so that gameplay will not be disrupted. It also prevents customers from creating "throwaway" accounts to harass others.
Of course, please keep in mind that you are not billed for the first month of the game. Thus, if you want to set up your account immediately, you can use a credit card. Then you can easily change your payment method at any time during your free month, before your paid subscription starts.
NOTE: If you initially set up the account with a credit card, but want to switch to use only a prepaid game card, you must cancel the credit card subscription manually on your Account Management page. Otherwise, when your prepaid game card runs out, the credit card subscription will begin and the credit card will be charged.
Account Creation FAQ: http://www.blizzard.com/support/wowbilling/?id=abl01668p
Account Management Page: http://www.worldofwarcraft/account/
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