How to verify payment info on itunes with a giftcard?

Whenever I attempt to buy an app on my iPhone it will ask me to verify my payment info. I already have 30 dollars redeemed on my account so how do I verify it even though my giftcard
is already redeemed?

They just want to make sure that you are aware if it being a paid app and that you know it is going to get billed. Just in case you click purchase by accident.