iStudiez Pro is free to use without the Cloud Sync service.

You may like to buy the Full Version of the app only if you want to sync the app's data.
It should be purchased on every platform where you'd like to sync the app's data.

It's not the obligatory payment.

The payment we get from the in-app purchase is used to rent servers and pay the salary to our devs, support, marketing department, and other Team members. 

Thank you for understanding.