The normal case is that payment should be taken from the customers who owe these debts without any additions. It is not permissible for you to add the lawyer's fees to the customers' as you preserve your money and your interest. This would be considered Riba (usury/interest) which was practiced in the pre-Islamic period and this case is
additions made due to delay. Since it is stipulated in the contract which was signed by both parties [the car rental agency and the client who rents the car] that the fees would be paid by the car rental agency, it is not permissible for you to take the fees from the customer as this is a Batil (null and void) condition and should not be considered. (Part No. 13; Page No. 337)
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.