Request for Assistance with App Subscription Billing and API

[dfbxfdre]

Hello,

We are developing our app subscription and encountered some issues while reading your billing requirements and testing the API calls. We hope you can help us resolve these.
The scenario is as follows: we only offer two paid subscription plans, plan A = $299/year and plan B = $1999/year. Suppose a user has already subscribed to plan A and, after several days, upgrades to plan B. According to your billing requirements, we should follow the proration logic (Refer to: https://shopify.dev/docs/apps/launch/billing/subscription-billing#proration).

Now, we are facing the following issues:
1. Where can we obtain the actual charge amount for this upgrade? Is there an API that provides this charge information? Currently, we can only see it from the interface (see attached files), and there is a delay.
2. Is this payment amount calculated by you Shopify platform? What is the calculation formula? For example, (planB amount-planA amount) or (planB amount-planA amount)*(cycle_days_left / total_cycle_days ) or else.

We would greatly appreciate your guidance on these issues.