Different countries in the world have different VAT rules. In order to be compliant, VAT must be included for customers in countries that charge VAT. This could automatically be added to the subscription / invoice costs if the customer provides their country of residence.

Ideally there would be two different options to add VAT: Add it to the bill (e.g. $10 amount becomes $10 + 21% VAT), or deduct it from the received amount (e.g $10 amount becomes $8.26 + 21% VAT).