Charging for pick up after stay is booked.

How do you charge for a service that was added after the actual stay was booked. For example, I have a guest dog now whose owner went overseas and will coming back tonight, and she has asked if I would drop off her dog for her. Understandably, after a long flight, she's going to be exhausted and I live sorta far from her, so I don't mind dropping off her dog.

Has anyone done this before? How would I charge for drop off without extending the stay?