Avis International Avis Rental Car Rental agreement switched from Dollars to Europs. overcharged, Internet

In the summer of 2009 I booked three separate car rentals in Europe online with Avis.  The rentals in Rome and Paris were without problems, but the one in London was a nightmare.  I booked a car for approximately $250.00 online before leaving the US.  When I went to pick up the car in London, I was made to wait for over an hour and a half while the customer service reps laughed, took phone calls, and claimed to be "finding" a car for us.  I was asked if I wanted to be billed in dollars or Euros. 

I stated Euros, as I knew that the exchange rate my home bank would give me would be better than Avis's.  I was given no map of London so it took us another hour to find our way out.  When I returned the car at the airport four days later I was running late and quickly scanned the contract, saw 280.00, signed and gave them the keys.  Once we arrived home I found my credit card had been  charged over $500 for the car rental.  I reviewed my signed contract and saw that they had charged me 280 Euros, rather than the proper conversion of 250 Dollars to Euros, which would have been about 170 Euros at the time.  I disputed the charge first with my bank, who upheld it as an overcharge. 

I wrote numerous letters to Avis, with numerous people who were either too stupid to understand the error or intentionally obtuse.  I was finally told to contact Avis International corporate headquarters IN HUNGARY.  That head honcho stated I had signed the agreement and the charges were correct.  Master Card denied my dispute stating that I had signed the contract and so had to pay.  I feel this is an intentional fraud by Avis, to use currency exchange and online booking and fine print rental contracts to up their charges illegally.  I will never rent from Avis again.