The Organize Series core contains everything you need to have series on your blog/website and will always remain free. However, there have been an incredible number of requests for features that give an even greater amount of possibilities for how Organize Series can enhance your blog. Some of these add ons we've released for free, and some are available for purchase. Your purchase of these add ons help fund the development of Organize Series and keep it free for everyone. Thanks for your support!