How is iCloud billed?

Hi,

I just received an iCloud reminder email from apple:

"This is a reminder that on 03/12/2014 you will be charged £7.49 for your 20 GB storage plan using your MasterCard. To change or cancel your storage plan, you can downgrade it from your iPhone, iPad, iPod touch, Mac, or PC.*
The iCloud Team"

I have over £96 Apple ID Balance, why are they charging my cc instead of my iTunes Store credit?

Thanks for the responses, the reason I would expect icloud to payment to be taken from my itunes credit is because everything is billed through apple. For instance if I buy an app from the App Store it's billed first from your available iTunes's credit any failing that from your credit card on file. If I make an in app purchase it does the same thing, available credit > credit card.

iCloud should be no different, when I upgraded last year to the 20gb plan payment was taken from my available itunes credit, not my credit card.

A little update, iCloud was billed today and used my "store credit", ie my itunes balance and not my credit card.