User Profile

I am developing a mobile app that has a paid subscription. I am interested in triggering a push notification when users change their subscription status in the App Store - for example, if they were to turn off auto-renew on their subscription, that would trigger a push notification using APNS to offer a discount. How would I go about sending a notification to a specific user based on the Server Notification payload? It is unclear which, if any, of the response body payload can be used to identify the user. Furthermore, I need a way to associate this identification with a device token so I can send an APNS push notification to the user. I have had the notification toggle available on launch for a while. If I understand correctly, users toggling "Allow/Deny" for registerForRemoteNotifications() notifications only changes their local settings. My first instinct was to start storing user data when they consent or decline notifications on launch - I could store [some kind of user ID, notifications_allowed (bool), and the APNS device token]. That way, when receiving an App Store Server Notification, I can reference this table, then fetch their device token and send a push notification if they have agreed to notifications. However, this seems somewhat fallible because it doesn't track users who change their notification settings in their own setting later. To summarize, my questions are the following: How can I identify a user from an App Store Server Notification payload? How could I store an identifier along with the APNS device token when users consent or decline push notifications that can later be used to associate their device with an App Store Server Notification?