On the AppStore, every 1 transaction in 10 has a billing issue meaning that 10% of your potential customers risk churning (or not even becoming customers in the first place) without deciding to do so.
A research by Forrester indicates that the main reason for this “involuntary churn” is insufficient funds followed by card limits & expiration issues.
Fortunately Apple tracks those users and enables you to react to this situation. With the help of Push Notification Service providers you can easily leverage Apple’s tracking (called subscription events) to trigger your notification automatically. Leanplum, Firebase, and Clevertap are all good solutions to adopt.