You might consider using a Handling fee, which can be set as a percentage of the item total (pre-tax/-shipping), although this would require buyers to provide a shipping location (country & zipcode) in the cart and a shipping address during checkout, which may be confusing if you're not actually shipping anything.
Aside from that, we don't really have any way to accomplish what you describe; we could only suggest just including the booking percentage in the actual price of each ticket.