Are they specifying reusable, or just removable? Pretty much any vehicle signage, even most paint, is removable. If they're wanting both reusable and removable, magnetic is about the only way to acomplish it. Static cling might work, if they don't drive much, and never get over 30 MPH. Magnetics won't scratch the vehicle if they're properly cared for... but almost nobody cares for them properly and the results can be a lot worse than scratched paint if they are ignored. So the customer has a valid concern if they aren't willing to go the extra steps to care for them.
I hate selling magnetics, and make customers sign a waiver which both explains how to care for them as well as shows pictures of how badly they can mess up a paint job if the care instructions are not followed. I really hard sell permanent signage on a vehicle for anybody other than a soccer mom who's only selling real estate a couple of days a week to pass the time. If it's a construction contractor who only wants to be able to take his signage off for one week a year when he goes on vacation... I sell him permanent signage and a magnet flood coated in a matching color to his vehicle so he can cover the signage up for that one week.
GetNoticed Signs & Graphics
With the speed of technology today, anybody who says "it can't be done" is usually interrupted by somebody who is already doing it.