From our Billing FAQ:
http://wiki.spiralknights.com/Billing_FAQ
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Alas, due to issues with fraud and credit card abuse, we have a limit imposed on the total amount a player can spend without giving authorization to Three Rings. We apologize for any inconvenience this may cause for you.

Accounts start out with a spending limit of $100 for a 30 day period--not a calendar month, but the past 30 days. This limit will easily be enough for the vast majority of players, but some may wish to spend more for various reasons.

As you spend near this limit it will automatically go up to $200, $400 and $800 slowly over time.

Should you wish to raise your Spiral Knights account's spending limit ahead of schedule, you may contact us and we will review your account to see if this is possible. Please note that we are unable to raise the limit in some cases. In those cases, the spending limit will still rise normally over time while the account is in good standing.

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You can use the contact us link there to make a billing inquiry and they can help you from there :)

Hopefully this helps :)