Had to read the documentation to see that serving the comments-widget one time consumes 1 credit, but sending a single email-notification consumes 15.
It seems very "expensive" to send emails.
Looks like a cute product, and the pricing mechanism is interesting. Good luck.
(I wrote some a simple self-hosted system, far less featureful, but it seems popular enough - http://github.com/skx/e-comments/ )