If you are only returning it one day late, the Hertz rental extension number tells you just to return it, no need to 'file' for an extension. You will be charged the CURRENT daily rate that you are paying, NOT the going rate. For example.

I guess I am selfish. Right before I left Oahu before Xmas, my rate was 35 a day. I noticed when I came back on the 20th, my corporate rate was not valid for two weeks and the rate was 120 a day!! Guess what, I parked that baby, flew home, and flew back and will be returning it tommorrow, when my corporate rate comes back into play. Sorry, not paying 120 a day.