I will be dog sitting in their home from 8am Sat. to 2 pm Sun. Does just the overnight fee apply or should there be an extra charge?

That's totally up to you. One "night" of house sitting technically would be over at 8 am on Sunday. That's six hours over. I never begrudge a customer a couple of hours. Since you do drop-ins, why don't you just add in the drop-in for those extra hours?

Add: Instead of using the drop-in, just put a modification amount on the invoice to reflect the overtime hours. Then it won't be tied to the app's monitoring program.

Karen gave good advice but the additional fee should be added to the booking request before you and the client click and accept the booking. Best of luck