I picked up the car the next day and returned it on time. When I dropped off the rental, a representative said the computer system was down. But when I got my bill the next month, there was an additional $1,116 charge because of the late pickup.

I believe this additional charge to be unfair and am requesting a refund. I believe Alamo should honor its promise of the prepaid amount as well as the representation that my reservation would be held for 24 hours. Can you help?

- A.S., San Diego
Answer: So Alamo is charging you an additional $1,116, even though you rented one of its cars for one day less. Where's the logic in that?

The answer: It's airline logic. Air carriers have figured out a way to quadruple the price of a ticket depending on the time you bought it. So a "walk-up" fare purchased at the last minute can be hundreds, and even thousands, of dollars more expensive than an advance-purchase fare. But you're getting the same seat.

You prepaid for your rental weeks in advance - kind of like an advance-purchase ticket. But when you didn't show up as expected, the system reset your rate, charging you the more expensive "walk-up" rate.

The computer problems should have been a showstopper. When a representative says the system is down, you need to get a receipt, even when it's handwritten. That way, when the company tries to double-dip, you can dispute the charge using the invoice as evidence that your bill was paid.

Even when you have assurances that you won't be billed again, it's important to monitor your credit card. A car-rental company isn't going to send you an e-mail letting you know that you're being charged more. It's just going to quietly take it off your credit card. As soon as you see a questionable charge on your card, mention it to the company or dispute it with your credit card company.

I think you would have won a credit card dispute, by the way. You would have needed to show the prepaid voucher as proof that you had already paid for the rental, and the credit card company should have sided with you.

I suggested you write a brief, polite e-mail to Alamo, asking it to reverse the charges. The response? A snippy note that said, "Any changes to the basis of the reservation makes the rate subject to change," and adding, "We are disappointed that this policy is a source of dissatisfaction for you." Alamo denied your request.

Like you, I was unhappy with the denials by phone and e-mail. So I contacted Alamo for you. It promptly refunded the $1,116 it should never have charged you in the first place.