When you click the "Add Fuel" button you will need to first select your fuel equipment. Once your equpment loads you can enter your meter readings to calculate the gallons sold and add it to the invoice.
Fuel discounts and taxes are all setup prior to the point the ticket is created, so by this time the price will be automatically calculated based on a variety of factors. Various disocounting methods are supported and will be identified by things like the customer code, the tail number, the make and model of the aircraft, the product sold and even it's quantity.