Hi Euge,
Yes we are going to be updating the docs soon to better communicate what happens to the trial days when you create, upgrade or downgrade charges, thanks.
Hello,
Billing Webhooks are only available through our Billing GraphQL API, you can find more information about this here and how to create and subscribe to them
https://shopify.dev/tutorials/bill-for-your-app-using-graphql-admin-api
