This has happened to me at least ten times this year so far with the Amex Plat rate. It will charge you the incorrect rate when you pick up the car if you don't get ahead of it. Before they print out the rental agreement confirm the price with them if it doesn't match your original email confirm have them "click into" the reservation to review the booked rate. My booked rate always shows up correctly on their screen when they do this it just doesn't match when they print the agreement. They usually need to do a manual adjustment once they see the other rate. The other thing I have done a couple of times is just call into customer service after the rental is done and they will credit you the difference without any issue. Sometimes I feel dealing with customer service is easier than dealing with some of the newer people at the offices the credit posts back to the credit card in a day or so.