Supporting SKPaymentTransactionObserver for Flutter app

We are interested in supporting App Store Promotion for our subscription feature and noticed saw that SKPaymentTransactionObserver method is required to process this transaction.

We are using Flutter with RevenueCat (and inexperienced with native iOS) and had some clarifying questions:

1. Is shouldPurchasePromoProduct in https://docs.revenuecat.com/v3.0/docs/configuring-sdk equivalent to supporting the SKPaymentTransactionObserver?
2. If question above is yes, does shouldPurchasePromoProduct have to be implemented on the native iOS side? Is there any recommended ways to support this in Flutter? 
3. The documentation mentions “If your app is able to handle a purchase at the current time.”  What conditions or user states should we be aware of to know if the purchase can be handled (e.g., is it just whether we can tell our app that the user is now a subscribed user?)

Thanks!