onPurchasePromoProduct
abstract fun onPurchasePromoProduct(product: <Error class: unknown class>, startPurchase: (onError: (<Error class: unknown class>, userCancelled: Boolean) -> Unit, onSuccess: (<Error class: unknown class>, <Error class: unknown class>) -> Unit) -> Unit)
App Store only. Called when a user initiates a promotional in-app purchase from the App Store. If your app is able to handle a purchase at the current time, run the deferment block in this method. If the app is not in a state to make a purchase: cache the startPurchase block, then call the startPurchase block when the app is ready to make the promotional purchase.