I once booked a car from National at MQP. To my surprise the full rental amount was immediately charged to my credit card even though this was a flexible booking. It freaked me out a bit as I have never had this happen before. Even worse, there was no way to cancel the booking on line and I had to call their offices to get it cancelled.
Turns out they are a franchise operation using their own dodgy practices even though the booking was done on the National web site. Took ages to get the refund. Wouldn't touch them again.