Do we need to create some additional API calls or do we need to provide some sort of UI or hosted service for constant auth?

  1. You only need additional API calls. You will need to do few API calls to get Token and refresh Token.
  2. When you have the Token, you will use it for every request to the Subscriptions Integrator API or the Invoice API.
    1. You'll find the API here 

You don’t need additional services hosted. What you need is an additional endpoints in your service that will handle a redirect from our system with auth code after merchant grants access to your app.