Trying to dig up some info regarding this, as I was prompted by my publisher to look into it:
It’s a new beta feature in iOS 11 that allows users to start a purchase outside of your app and complete it in-game once loaded. The functionality seems complex, requiring the developer to handle a purchase request at any point in the game’s life cycle.
Just wondering if there’s any word on this being integrated into Unity’s official IAP implementation.
I’m also looking for this feature. Unity guys, any news?
I think this has been added to the Unity IAP update today 
Thanks @stevenatunity !
Important question about the new feature, i guess that when user buy the promotional iAP we get in the the “ProcessPurchase” callback when app start. But how do we get indication that this item was promotional iAP and not regular iAP (we use the same item id) ?
Hi @stevenatunity !
Why do you think so? Did you test it?
By the way, i encountered a problem with the promotional IAP (thread: [Solved] iOS Promotional IAP only working when the app is already in the background - Unity Services - Unity Discussions)
Is someone can contribute to solving it i will be glad.
It’s in the release notes. Haven’t tested it yet