Cingular/AT&T upgrade a rip off?

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So I went to upgrade my phone today since it says I am eligable for an upgrade to both of our lines. So I pick a phone, put it in my cart and go to check out. There is no fee's other than an $18 upgrade fee that is says is applied to next month's billing statement. So it says total due is $0.00 for my phone, and I go to the final checkout page and it wants my credit card number! I am leary to do this since it says the total is 0. Does anyone know why they ask for this if there is no charge? I'm afraid of giving them the number and somehow being charged the full amount for the phone.