How does the one-time plan work?
After the free sign up, you can purchase a one-time plan for processing you images. You can use it for bulk optimizing your past gallery, or for optimizing new images uploaded to your website. The process is different because you pay for it only once. This plan is recommended for websites that have a large gallery that needs to be optimized.
Please note that one-time credits never expire so you can use them until all of them are exhausted.