The Meteor (metered) subscription requires a $0.99 cent payment up front, and bills for what you use during the month at a rate of $0.003/image. Every image optimized counts, including resizes generated by WordPress, so remember to check the Bulk Optimize page on your WordPress install to see how many resizes you have on your site before selecting a plan.
No charges are made until an account reaches 3,000 credits or more, so if you do not use ANY credits in a given month, you will not be charged at all. The $0.99 fee will give you 330 credits to start with.
This plan is no longer available, you can purchase credits bundles and setup auto-recharge instead.