Your My Account page lists the photo credits you have and the date they expire, if they expire at all. You can apply these credits to future orders until you use them up, or until they expire, whichever is sooner.
Photo credits automatically apply to your cart when you add the applicable product. If your total doesn't reflect the credits applying to your cart, make sure you have met the product requirements for the credit. You may also need to click the link Apply underneath the Order total. If your cart still does not reflect the credits, please call Customer Service at 866-264-2910.
NOTE: If you have credits in your account, they will automatically apply to the correct products in your cart. There is no way to avoid using them when checking out.