I’ve found this relevant item in the Stripe help: Charging sales tax, GST, or VAT on payments : Stripe: Help & Support
When making charge requests, Stripe does not automatically calculate any tax information for you. The amount you specify must be the total amount to charge the customer, inclusive of any fees or taxes that they should be charged. This will need to be done manually, or you can build support to include taxes in your charge request amounts in your integration’s backend.
So, the total amount has to come from the Ghost side, it looks like.
And, maybe publications are VAT-exempt, but I don’t think that’s the case with HST, unfortunately. As long as a business’s income is under $30K per year, we don’t have to charge it, but once we go over that magic number, charging sales tax is required. I hope Ghost is looking into integrating this feature! It’s pretty necessary on the global scale.