I'm curious how you were able to get a car without signing a rental agreement. Even if you're a Blue Chip member (express service), you should still have to sign it.
Technically, if the rental company doesn't have a signed rental agreement on file for you, you could probably dispute the entire rental charges and not pay a single cent--though that wouldn't be terribly ethical.
Regardless, your original reservation should carry some significant weight if you complain to the national/international corporate HQ, since the reservation is your "agreement" with them, versus the rental agreement, which is the agreement you have with your renting location. Corporate HQ will usually side with their own agreement over a rogue location's agreement when the two conflict--you can claim that it's an error in their computer system that the correct kilometer cap was not transmitted with the reservation and therefore you should not have to pay.