Each Challenge you create will be charged at $2.29/user. When your challenge is over, you will be sent an invoice for the number of users who participated in the Challenge. You can pay by credit card if that is convenient.
If you find that the options available in Standard are not custom enough, or you'd like Concierge Support to assist you with your Challenges and initiatives, upgrading to Pro may be a good option for you. Learn more about Shine Pro here.