Application Charges Billing API

[MuhammadDaniyal]

Hello, I have implemented the application charge in my app. Merchant can purchase credits of 40$,50$, 100$ etc for the future purchases in the app. The Shopify review team tells me that:

"Must implement billing API so it can accept, decline, and request approval for charges again on reinstall. Resolve any errors occurring in the billing modal during the billing flow."

My logic is when merchant purchase the credits I am storing those amount on behalf of shop and it can be deducted when merchant uses my services. What to do if merchant uninstall my app and reinstall it? 

Should I remove the credits of the merchant to 0? 
Right now, I am keeping them as it. Because, when the app reinstall so the remaining credits should be there?

Kindly guide me about the flow of application recharge Billing API.