How is tax charged on my subscription?

Plan prices are shown before tax. Your plan is part of your Shopify bill, and Shopify adds whatever tax it must charge in your region. nouz adds no tax of its own. A payment that fails is handled the same way, as part of that bill.

VAT is the sales tax of the European Union. For a business, whether a VAT ID changes that tax follows Shopify's rules, and the ID is kept in your Shopify account, not in nouz.

The tax appears on your Shopify bills, beside everything else Shopify bills you for. That is also where you find the full amount of each charge.