I extended a rental by a day (and changed the return location) about 2 weeks ago.

Before making the change, the phone agent advised me that they would be authorizing an additional $X on the card due to the extension.

On my receipt for the rental it shows :

MISCELLANEOUS INFORMATION
CC AUTH: 15233P DATE: 2018/08/11 AMT: 239.00
CC AUTH: 15233P DATE: 2018/08/11 AMT: 78.00
CC AUTH: 15233P DATE: 2018/08/12 AMT: 39.00

I'm not exactly sure what to make of those amounts. The first is when the car was picked up, and is for the estimated 1 day rental cost + $200.
The second is when, later that same day, I called and extended/change the return location. Amounts seems to be for the new 2 day rental cost, without any extra.
The third line is the date I was originally supposed to return it, is for the the 1-day rental rate...

To be best of my knowledge these aren't cumulative (given they have the same Auth number), so I have really no idea exactly what was going on there... The amount I was finally charged was correct, and was just below the $78 amount.