Site credit and gift cards do not expire. They can be combined with credit card purchases, promos, discount codes and may be redeemed for anything on the site. You do not need to use the entire balance in one purchase.
To redeem, enter it in the section at checkout "Gift card or discount code" (see photo). You may combine discount codes with your site credit or gift card. Simply enter them one at a time and hit "Apply". You should see an itemized breakdown of your redemptions.
Any balance remaining will stay on your gift card or site credit. Gift cards and site credit balances cannot be redeemed for cash and have no cash value.
If you made a purchase with a gift card or site credit and a refund is issued to you, it will be sent to your original payment method (whether you paid partially with gift card or a credit card).