The problem, of course, is the signed contract. If you dispute it with your credit card, the car agency can just provide them with a copy showing that yes, you did agree to the extra charge. If it was a charge that wasn't on the contract, then yes, a dispute would be easy. And the contract also explicitly says that verbal agreements are not valid.

The whole scheme depends on you not reading all the terms and relying on what the agent tells you, but that's not how the contract is written. The company can take it up as a customer service issue, since they (probably) don't want agents scoring extra commissions this way or customers posting online about it, but legally I think you're stuck if they won't voluntarily refund it.