The functionality of universal purchases, announced in the last WWDC, has taken to be available, more or less the same time as the possibility of sharing folders in iCloud ... The important thing is that it is already a reality and it seems that it was a function that the developers were waiting for, and each is greater number that has adopted the universal purchases.
Universal purchases within Apple allow users to pay one time for the iOS and Mac version, without having to buy the Mac version independently or vice versa. The latest application to adopt this universal shopping system is GoodNotes, the popular note-taking application on the iPad, and it is also available for the iPhone and Mac.
So far, if we wanted to use the version for macOS, synchronizing the data with the application for iPad / iPhone, we had to pay 8,99 euros in the Mac App Store. After the last update, we just have to buy the version for iPhone and iPad so that it automatically appears in the Mac App Store as we can download the macOS version for free.
In the case of GoodNotes, according to the developer, Apple has not allowed it to merge the existing version on macOS with the one currently offered on iOS, so if you are users of the version for macOS, you will have to download the new version (The version available so far has added the Legacy sign indicating that it will stop receiving updates).
We only found this problem in the macOS version, not for users who had previously purchased the iOS / iPadOS version. However, if you had just purchased the app for macOS, the app will continue to work without any problem but it won't get any update. If you want the application to continue receiving updates, you will have to go to checkout and pay the 8,99 euros that the application costs in the Mac App Store.