Insurance company paid only part of car rental charge

My daughter was in an auto accident where the other driver was found to be fully liable. At the time, his insurance company authorized 6 days car rental at a specified rate. I contacted them as soon as I heard from the repair shop that it would take much longer than the 6 days authorized and was told not to worry, to just submit the receipts when all repairs were complete and they would pay for the rental.

Well, it ended up taking 25 days to repair the car, and after submitting the car rental receipt for close to $900, I received a check for just $300. I sent a letter over a month ago requesting clarification, but have not received a response.

Since it is such a small amount, I was thinking about small claims court, but am not sure what the law says about these type of disputes.