You can modify the booking fee for the entire account in the configurations.

1. Go to System Setup -> Configurations.
2. Scroll down where it says Order Settings. 
3. Enter in your desired booking fee. This will apply to all trips within your account.
4. Save.

If you want to modify the booking fee for specific trips.

1. Go to System Setup -> Trips.
2. Go to the General tab on the left and scroll all the way to the bottom.
3. Enter in your desired booking fee. This will apply only for that particular trip.
4. Save.