Out of interest, what did you pay for those 4 days and what time did you arrive?
I tried making a dummy booking on the website for 4 days in November, I got prices ranging from 11€ to 97€ with Hertz.
The Hertz rate clearly specify it includes airport fees/surcharges, the one for 11€... well, you get the picture.
Always always read the fine print about what is and what is not included in the price you get.