Yes. You should always mention what add-ons you want when you are making your reservation with a rental car agency. If you wait until you arrive at the counter to mention an add-on, the agency might not have the add-on available for you at that time. For instance, according to Minnesota law, any child seven years old and younger or shorter than 57 inches must be in a child safety seat appropriate for their size while riding in the vehicle. So, if you need a child safety seat, you need to reserve the correct size - infant, child, or booster.