I must admit I have sympathy with the poster.
Recently we went to Ireland for a weekend, and we hired a car for the journey from Dublin Airport into the city centre, and back again. The cost online, was £40. Which I was told I'd pay at the kiosk within Dublin airport.
When we arrived, I gave them my printed details, I was then asked if I wanted to upgrade the insurance (Which doubled the cost to £80) and for peace of mind I agreed. I was then informed that I would be charged for a full tank of fuel, with what I don't use when I return the car, paid back to me. Incredibly, this came out at a total of over £160. Four times my original quote and £80 for a tank of fuel alone! Stupidly, I didn't notice how much this was at the time, it was only when I received my credit card bill that I realised, and noticed that they had not refunded me any petrol (The car had only been driven to from the airport to the hotel and back again!)
The good news, was that I sent a very strongly worded email to the company involved expressing my disgust at the way they had overcharged me for the fuel (The Car I rented was a 1.4 litre engine similar to my one at home, and I know the maximum it takes to fill it, even from near empty is £50). I did receive a £74 refund (Which meant I had used £6 of fuel, which is probably near correct) and most amazingly of all, an apology! However it still irks me that the quoted price of £40, was in fact over double that when you take into account insurance and fuel costs.
However the same tricks are used the world over. At Sanford Orlando, if you hire a car, you are often offered, many times over, the chance to upgrade (at a cost) to a better one, and then if you have the "all inclusive" insurance, you get offered extra tyre insurance as tyres aren't included! (Which makes a mockery of the All Inclusive tag).
Personally I think the vagaries of the insurance market allow car hire companies to use this scandalous practices to procure yet more money from the hirer. The time has come, in my opinion, for your own car insurance to cover you fully comp, for hire cars. That would solve the problem once and for all.
Ian