I want to write an app, which enables the user to calculate financial credit plans. The users will be professionals in the financial sector. The idea is that they pay for each month of usage, basically a subscription. The app will not have any free features, so the subscription does not "unlock" premium features, instead the subscription "unlocks" all functionality of the app. Can I use In-app non-renewable subscriptions for that? Can someone point me to a good tutorial?
Yes you can use the non-renewing subscription option for your application.
Reference tutorial link
If you would not like to provide any functionality free then I will suggest to make your application as paid application. Otherwise maximum chances are there for your application to be rejected.