How do I add an automatic fuel surcharge?

Navigate to Settings -> Billing Hub -> Product & Service Items

Click Add Item -> Fill in Needed information, description, account and Price.  You can add Taxes if applicable.  Click Save

After clicking "Save", navigate to to any of the Product or Service Items that you want to add the Fuel Surcharge to and click "Edit".

Scroll to the bottom of the screen and select your new "Fuel Surcharge" from the Fuel Type drop down.  Then you will be able to select how many units you would like to charge with each rental.  Under the "Apply To" drop down, you can select the Rates that you would like this Fuel Surcharge to apply to.  Click "Save".  This will then add the Fuel Surcharge automatically to the invoice moving forward.