my husband booked dollar rental car through carrentals.com and got a great rate. It did state that you had to show proof of insurance if you waved the CDW. The proof would be a letter from your Credit card company stating that they are providing you with CDW. We called TD Visa and they send us the letter we requested ( full coverage with no deductible) which they knew about and had no problem issuing.
When my husband got to the counter they did not accept this letter saying that the letter needs to come directly from Visa and not from our bank issued Visa. Husband tried to explain that this letter from TD Visa is indeed issued by Visa but they would not accept it and my husband had to add the CDW to his rental contract and it was very expensive and also if something was to happen the deductible would be 600 or 900 euros ( sorry don't remember exactly)
Also be aware that Dollar/thrifty rental desk is in Terminal 2, so if you are landing in Terminal 1 you will need to take the airport train over to terminal 2. It is only inconvenient if you have more luggage with you since you are not allowed to take the luggage cart on the train and therefore would have to unload your luggage and carry it into the train.
After this experience my husband said he would not rent with them again since the price compared with Avis and hertz is not that much cheaper when you have to add the CDW because they wont accept your credit card rental car insurance. We never had any issues renting with Avis or Hertz but can not say the same about Dollar/Thrifty.
Hope you have better luck.....