In the Windows Store for Windows 8, are my in-app purchases fully available for users of the trial version of my app? If not, will they be able to purchase the in-app purchases while still being on the trial version?
From here:
Note: A customer can only make an in-app purchase if they've purchased a full version of your app.
This doesn't prevent you from making your app free and then limiting what they can access until they make an in-app purchase to "upgrade" to the "full" version.