Question about the implementation of purchases in the Android application?

Question

Android

the application is free, but the user can buy additional functions, packages for 3 months, 6 and a year.
as I understand the principle, I need to store a variable that is false at startup, after startup I do QueryPurchases ( developer.android.com/google/play/billing/billing_... ) and see if any package has been purchased, look at its date purchases, if the corresponding period (3-6-12 months) has not passed, then I set the variable to true.
if the variable is false and the user makes a payment, after the payment has passed onProductPurchased - set to true.
those. the state of the variable is not stored, but is determined at startup or during the operation of the application, and based on its value, we limit the functionality