After you're done shopping, head to checkout. When you get to the payment section, click on "Uncommon Credit or Gift Certificate"
When this section expands, you can enter your credit or GC number and click "Submit". When you're finished, you can pay the balance with your credit card or PayPal. To enter multiple GCs, submit them one at a time. Uncommon Credits cannot be “stacked,” meaning only one can be used in a single purchase. It is possible to use one credit alongside a GC.