We are happy for you to play with the API. You can charge for your tools or not, show ads or not, or do what you want with it, so long as it does not do anything deceptive, manipulative, steal users' contact/expense data, or something we think violates the spirit of fairness, transparency, and/or awesomeness.
We do reserve the right to revoke your API key if we think you are doing something mean or bad for our business. We might need to throttle your API calls if they are not optimized well, or for some other reason, but if so we will warn you.
All data must be stored locally (ie, on the user's client, not your app's servers) for now. You agree you won't use the contact info or expense tracking data outside the app - this is important for protecting our user's privacy. Violating this will result in immediate termination of your API access.