Overpaid for dog sitting?

I watch my neighbor's dogs (they have 2) and i watched them for two days from 9:00am-3:00pm and i'm not the person that lets the dogs out and lets them back in I give them baths, take them on walks, paint there nails, throw toys for them and stay over there 9:00am-3:00pm and I also watched them for an hour one day and I charge $15 dollars a day and that would be $30 and the one hour was nothing so i didnt really need to be payed for an hour and she left $50 for me on the table so what should i do give $10 or $20 back? i feel bad and like she over pays me