my find documents(app store server notifications and app store server api), not find next billing date description, i want new subscription and renew subscription send email to customer, context contains next billing date

I have an APP with its own user system. I configured subscriptions in the Apple Store and divided them into levels 1 and 2. User logs in to the App Store with apple id A, then logs in to my APP with account A1 and subscribes to level 1 subscription. At this time, he switches to the App Store, switches apple id to B, then returns to my APP (the APP user is still A1 at this time), finds the previous subscription and upgrades it. At this time, apple ids A and B will be charged for subscription at the same time. I need to restrict this operation. My solution is to obtain the user's current valid subscription from the App Store when upgrading. If it cannot be obtained, the user is prohibited from upgrading. However, I only found "Restoring purchased products" from Apple's documentation. This document only explains how to restore completed transactions and cannot be called frequently. Are there any other solutions or APIs?

I have an APP. After User A (Apple ID) subscribes, the user switches to User B (Apple ID) in the App Store. I want to cancel the subscription. If the user switches back to User A (Apple ID) in the App Store, I want to restore the subscription. How can I achieve this? According to the official documentation, it is not recommended to call the restore function frequently.

For example: The transaction id obtained by the app is: 2000000625603237 The transaction id notified by Apple is: 2000000625603236 Why does this happen?

When my app was tested on testflight for subscription, I never received the payment email. Is it because of the sandbox environment? Will I receive the payment email if I subscribe and pay after the release of the app