This isn't a difficult problem. You give your car-calling app a "reject" button, which will send the car back to a central location for cleaning, and send you a new car.

Yes, and before the cleaner starts, a picture is taken, that generates a $100 charge to the previous customer for every separate bit of garbage.

A slightly customer-friendlier option would be to take the picture every time the car is vacated, and then clean (and charge) when the detritus reaches a certain level. After all it wouldn't be right to penalize people who are simply in too much of a hurry to care about previous travelers' candy wrappers.

Same way potato chip factories can sort out bad chips faster than the eyes can see: machine vision. A couple of cameras inside the car can take a picture, compare to a sample of a clean car and identify extraneous objects. Smell can be automatically checked as well.

A combination of computer vision, periodic inspections, feedback from customers, plus stiff penalties for abusing the vehicles would be one way.

If the vehicle is detected as being "messy" it can swing back to the depot for a cleaning or have someone dispatched to clear it out.