Well I don't know how one can considered whether 3 hours is a small or large delay. Since in Europe you have a 29 minutes of "grace", passed this time you are considered late and contracts always explain this clearly. The consequence is an extra charged day. The fact is that in Spanish market, Hertz charges a day more. It seems that on French market an extra day after the 4th one is consider as a full week.
So I don't think it is a matter of how many late hours. It is about the consequences between being charged 166 euros or 563 euros.
Especially that this particular Hertz office closes at 6 pm (6 pm was also my assigned time for bringing back the car) and that I asked if I could give back the car later than this time. The answer was yes but charged for an extra day, they never mentioned that it would be an extra day which in reality would turn to be a full week with more than triple price.

I do think there is a disproportion between my tariff and the tariff they charged me. I don't want to fight about the fact that they had to charged me an extra day. It was clear for me that I would be charged for this delay. However, I was not aware about the tarification of French market. Meaning that there is not 5 days fare, but only 4 days fare and full week fare. Who could knew this, especially that in Spain, where I am resident, tarification is different. And who could beleive that the fare for a week would become more than triple.

Understand that a small-budget family renting hertz for 4 days would got charged outrageously and without previous warning, eventually puting them in financial troubles.

In my opinion, there is an abuse and a lack of information from Hertz.
This is not being charged for your uncomplishement of the contract, but this a "guillotine", a real punishment. In this particular case Hertz treats me not like a customer but like as a child that need to be blamed.
You don't get charged for a service. You get charged for a punishement that is higher than most of fines you can get from Police on European Roads. Who get a fine in Europe for 400€? Maybe being drunk. Hertz, by charging its big fine is encouraging customers to drive unsafely, out of the law, to bring back the car on time.

Another thing is that, even bringing the car within the 29 mn of grace, this Hertz office would have been closed. It means that this car would not have been cleant and re-rented the same day but only the day after. There is no loss of profitability, no loss of vehicule availability for the next customer, even if the next customer wanted the car early the day after, cleaning service start early than 8 am and would have got the car ready on time.

Who would go back to Hertz with this fear to get charged triple. For information I rented Hertz almost 20 times cars in 2006, and always for personal use. I do know what is a hertz rental in Europe.

Finally I think that the negative response from Hertz customer service to my request for correcting this invoice is an unfair, inappropriate, unrespectful, anti-comercial answer.
I will intent a juridic procedure to at least force them to admit that consequences of a delay should be clearly announced to the customer. A customer well informed is always much better than blind customer discovering consequences at posteriori.

Jer