There is no possibility to bundle the purchases from different Stores.
Thus, the app should be purchased separately for each OS where it is going to be used.
Sometimes the App Store may "forget" that you have already purchased the app and it shows you "Buy" button.
If you use the same Apple ID to download iStudiez Pro (e.g. on your another iOS device) as the one that was used to buy the app, tap "Buy" button and you won't be charged again.
Otherwise, please contact Apple Support Team.