Many banks will block their US customers from making international purchases as a form of Fraud Protection. It's a great idea, but companies like us (Design Bundles and Font Bundles are based in the UK) can get blocked as well.
Please contact your bank to ensure your card is enabled for international purchases.
After you've made this request to your bank, please try the payment one more time. If it still does not process, please contact our Customer Support team, and let them know the last 4 digits of the card you are trying to use.