How to add payment for additional days?

First of all I'm charging 30/night which in my opinion should be 30 a day not night. I just recently booked a stay for 4 days and they wanted me there at 8 am ( I had to leave my house at 7am) and I didn't leave the 4th day until 7pm. So its like I only really got paid for watching the dog 3 days and the 4th day was free. Anyway because I had to stay later the last day, the lady whom I was watching her dog said she would pay me a little more for staying longer as she originally told me I could leave at noon on the last day. So my real question is how do you charge the extra fee for staying longer. I know you can charge extra things before you book the stay but it wont let me do that now that the stay is over. Can the client add addition payment to what she already paid.