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 f . .. More

Different types of extra cost to the total billing can be added, like you can add

10% VAT, 2% service charge, 5$ booking cost e.t.c.

To add these extra cost and taxes please login to your hotelier control panel and click on "Add taxes / extra:" section.

Regards
Guestjet.com support Team