Hi,
You'll have to calculate it yourself in the view of the invoice with some custom PHP.
It's not easy to do if you're not a developer bu if you're one, it shouldn't be a problem and you can edit the file "invoice" of the view "order" via the menu Display>Views in order to add your custom code.
Otherwise, you can contact our partners for a quote on that :
www.hikashop.com/home/our-partners.html