Unable to Enter or Change Credit Card Details via Customer Center or Web Store Checkout

Make surethe Hide Payment Page If Order Total Equals Zero check box isnot marked under Setup > SiteBuilder > Set Up Web Site > Shopping tab> Payment Page category. 

If theabove preference is checked, once the order total in the Shopping Cart isequal to zero, the Payment Information link will behidden.  If the above preference is not checked, regardless of theorder total in the Shopping Cart, the Payment Information linkwill always be shown in the Edit Your Profile page in the CustomerCenter or Website.