I had a similar problem this weekend with National. Rented a car one-way, picked it up at Emerald Aisle, and showed up at my destination where they wanted to charge me over $250 (base rate was $58)!! According to them, and the computer, I owed an addition $0.40 per mile. I adamantly refused to pay:

• Rental agreement said "Extra Time & Distance $0.00 $0.40"
• Rental agreement noted $58 was a "flat-rate"
• Reservation line, when I paid the reservation, indicated that there would be no additional charges or one-way fees associated with the rental.

After digging our heels in for about an hour at the desk, we finally got ahold of a supervisor at our origin (no supervisor available at destination, reservations line could not make changes, customer relations was closed) who agreed to waive the charges.

What fun that was...