What is the “FF surcharge” on my car rental receipt for?

“FF Surcharge” stands for “frequent flyer surcharge”. This is a fee of about $0.75/day that is charged any time there is a frequent flyer number entered into the car reservation. It is easy to avoid – instead of entering the frequent flyer number you want the car credit to go to in the reservation, just allow the miles to go to your car vendor number and then transfer them to a partner air account.