I emailed customer service and received this response. I've attempted these steps and it still doesn't work. Adding it here. I hope it works for someone.
Hello
***,
Judy here from the iTunes Store. I understand that you are having trouble with your credit card not being accepted. I know how frustrating that can be especially since this is your first purchase. I will be glad to help you in any way I can.
***, Apple has instated a new policy, in which customer are asked to enter the security code of their credit card when they sign into the iTunes Store on a new device (computer, iPhone, iPod touch, iPad). The is to prevent hackers from making unauthorized purchases on accounts. As you update the
OS on your iPhone, this made iTunes recognize the phone as a new device.
Once you have entered the security code for a device you should not be promoted to enter it again every time you sign into the iTunes Store as you have been.
Many customers have resolved this issue by temporarily removing their credit card information from their account. Here are the steps:
1) Visit the Apple Store online:
http://store.apple.com/us
2) Click Account in the upper-right corner, then click "Change account information" and sign in using the same Apple ID and password you use for the iTunes Store.
3) You should see your account-profile page. Under Method of Payment, remove all of the characters from the Card Number field. Do not change the expiration date or credit card type.
4) If you haven't already, select where you primarily use your purchases from the pop-up menu near the bottom of the page.
5) Click Continue. If you are asked to enter any other information, be sure to click Continue after making the updates so that your changes will be saved.
6) Make sure you're using the latest version of iTunes. It can be
downloaded free of charge:
http://www.itunes.com/download
Note: Installing the latest version of iTunes will not affect your library or any items in your account that you haven't downloaded.
7) Click this link to open iTunes and access the Apple Account Information page:
http://phobos.apple.com/accountSummary
8) Sign in to the iTunes Store using your account name and password.
9) On the Apple Account Information page, click the Edit Payment Information button.
10) Enter your payment information and click Done to save your changes.
***, the main reason customers' credit card information is not accepted is their billing name or address does not match the address listed on the credit card statement. Itunes is very sensitive. Any discrepancy in punctuation, abbreviation, or capitalization can cause errors.
For example if you listed 1st Ave. when entering your billing address, and then entered First Avenue when entering your credit card information your card would not be accepted.
If you moved, or if your zip code changed recently, ask your credit card company to make sure its records are up to date and match exactly what you are entering as your billing information.
To ensure your account is functioning, try downloading a free item from the iTunes Store. To do this, navigate to the iTunes Store main page by clicking the Home icon near the upper-left corner of the window. Scroll down and click the "FREE ON iTunes" link near the bottom of the main page, then select an item to download.
I am very sorry for the inconvenience this has caused
***, and I will do my best to ensure the issue is resolved. If you have any questions, or continue to have trouble, please don't hesitate in getting back to me.
Have a lovely week!
With Warm Regards,
Judy
iTunes Store Customer Support
Please note my hours are.
Saturday August 14th, to the 18th, 8:00 am to 5:00 pm. August 19 & 20, off. August 21st 8:00 to 5:pm.
August 22nd to 24th, 12:00 pm to 9:00 pm. 25th & 26th off. 28th & 29th, 12:00 pm to 9:00. 29th off. August 30th to September 3rd, 12:30 pm to 9:30 pm.
Thank you for allowing me the opportunity to assist you. You may receive an AppleCare survey email; any feedback you provide would be greatly appreciated.