It is possible to charge additional service fees to the ticket buyer. You do this by adjusting your organization's kickback service fee.
Click on the cog in the lower left corner and open: 'Organization settings'.
Click on the orange button at the top right: (Customize your organization name)
Go to Service costs
Check the Cashback service fee checkbox and enter the amount you want to add. These costs will be charged to the ticket buyer per ticket.
Do not forget to click on Save