Your photos won't print automatically. You need to manually send them to print. If you forget to select and submit your photos, they will not be printed.
What happens to my Add-on Prints credits? You're still billed for Add-on Prints on your subscription's schedule as long as your subscription is active, and each charge adds a credit to your account. Credits build up and can be used for 13 months from purchase. If you don't send any prints to print within those 13 months, the credits expire.
To get started, learn how to send photos to print in How do I send my Add-on Prints to print?