oStorybook is free, no financial contribution is required to use it.
If you really want to contribute to future developments then use the discussion forum to make suggestions for enhancement or comment on how the software might be improved.
If you are fluent in a language other than French or English you can also participate in the translation of the software, or just suggest corrections. To learn how, see the translation page. (link)
You can leave your comments and experiences on the Testimonials page.