The guideline says “cannot install code or resources in shared locations” - that’s probably the rule you’re breaking, as you said you’re opening the saver so that it gets installed, i.e. copied somewhere else.
Now, at least with other kind of helpers in an app bundle, they do NOT need to be copied (i.e. installed), because macOS automatically finds them inside your app bundle and can use them from there. That’s true for spotlight importers and login items, for instance.
Are you sure your saver can’t run from inside the app? Maybe you tried to install it on wrong assumptions that you need to do so? Otherwise, have you looked for help in the apple forum or on StackOverflow about embedding screen savers in your app?
Oh – but even if I am right - you don’t want that. You want to allow access to the saver only after the IAP. And that requires hiding the saver initially (so that it’s not automatically accessible) and then copy it somewhere else so that it can be used. Tricky.