Hi John,
Try logging into My 8x8 to update your payment information. You can get there by going to
login.8x8.com and then choosing My 8x8 at the bottom of the screen.
From there, you're presented to the main portal page. At the top choose Billing > Payment Methods.

Then add a payment method at the bottom of the screen. From there you should be able to buy items from the Shop page.