Travelers are responsible for paying all toll fees incurred during their trip.
If a guest doesn't pay a toll properly, they are responsible for the toll fee, as well as an administrative. More information about fees and fines here.
For all-electronic tollways with no way to pay manually (e.g., Golden Gate Bridge), the guest needs to reimburse the owner directly or inform Turo so the proper fee can be added to the trip. If the guest doesn't do so, this will result in an additional fee.