In almost two years of being a ZipCar client, I've never had any issues. Yes, the occasional coke can or candy wrapper left behind, but never a mess that I thought to be unreasonable.
If it turns out to be a bigger problem, install a camera that takes a shot when you take possession of the car, and when you relinquish it. Charge the renters credit card for messiness.
In almost two years of being a ZipCar client, I've never had any issues. Yes, the occasional coke can or candy wrapper left behind, but never a mess that I thought to be unreasonable.
If it turns out to be a bigger problem, install a camera that takes a shot when you take possession of the car, and when you relinquish it. Charge the renters credit card for messiness.