First, I am not licensed in PA and can only give you general advice. Generally, once the insurance company offers you the fair market value of your totalled vehicle, the carrier is no longer obligated to provide you with a rental car that is covered under your insurance policy. If you keep the rental car after the fair market value offer has been made to you, you may have to pay for the rental car yourself. Knowing this, you will have to weigh accepting the offer (even if you think it is less than your car is worth) against contesting the offer to see if you can get more. Usually the carrier will cut you a little slack if you have a reason for not returning the rental car right away. So put your reason in writing and ask them to work with you so that you are not out of pocket for any of the cost. Your carrier will probably pursue payment from the at fault carrier IF you are not responsible for the collision.
Sign up to receive a 3-part series of useful information and advice about personal injury law.