@Sandra1540 Unfortunately there is not a way to build optional add-on fees into the booking process. This is one of the features I'd most like to see Airbnb add - not only would it solve your problem, but it would also allow hosts to charge ahead for additional services like meals, airport pickup, and interim room cleaning.
Still, there is a workaround. If a booked guest wants to arrange an early check in or late checkout, you can go to Airbnb.com/resolutions and request payment of your specified fee. You can then make the arrangement conditional upon the guest approving the charge.