On the "Billing and Payments Options" page:
1. Under "Payment Method", click the little button for "Gift Certificate" as your payment method.
2. Paste your code in the box that becomes available. (Will be preceded by "Gift Certificate #")
3. Click the "Apply" button to apply the balance of your gift certificate to your order.
4. Any remaining balance can be paid with a valid Debit or Credit card, or Paypal.