In Add tax/ extra section . you can select % if you want to add your extra as percentage.
Suppose you want to add 10% vat on total then you can try like :
Service name: VAT
Amount : 10
Type:%

But if you want to add 10 dollar ( or your currency) on total then you can do like
Service name: Booking Charge
Amount : 10
Type: Flat fee.

Guestjet.com Support Team