I found two plugins one for iOS
and one for Android
that provides InAppPurchasing service. They seem to be often maintained and my impression is that users found them useful. Here is a nice blog
about the iOS one.
You can import custom plugins to AppBuilder as explained here
I think that your app should content the packages your are going to unlock using the InAppPurchasing. I am not aware of a way how you can dynamically update your app. Currently if you want to update an app package to a newer version you should publish the newer app package within the app store so I doubt dynamic update is an option.
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