On the "Billing and Payment" page at checkout:
1. Under "Payment Method", click the button for "Gift Certificate" as your payment method.
2. Paste your code in the box that becomes available. (Will be preceded by "Gift Certificate #"). Make you to remove any dashes or spaces from the code.
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.