There CAN be depending on the company, but it's almost impossible to enforce, especially given that all you give when you reserve the car is your name, email, and phone (make sure you don't put down any loyalty accounts when you make the reservation. They would have to reasonably prove that you reserved the car rather than someone who knows your name, phone, and email (such as an ex- or an angry neighbor), and it wouldn't be worth it to them for a small cancellation fee.