onPurchasePromoProduct
open override fun onPurchasePromoProduct(product: StoreProduct, startPurchase: (onError: (error: PurchasesError, userCancelled: Boolean) -> Unit, onSuccess: (storeTransaction: StoreTransaction, customerInfo: CustomerInfo) -> 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.