-keep public class com.android.billingclient.api.Purchase { public *; }
And maybe remove
### Billing ###
-keep public class com.android.vending.billing.IInAppBillingService { public *; }
-keep public class * implements com.android.vending.billing.IInAppBillingService { public static *; }
-dontwarn com.android.vending.billing.IInAppBillingService.**
And maybe remove