I'm not really sure what you're asking for. If you're looking for a system capable of accepting payment without an attendant there are many. Here is just one example:
I was thinking of a system that would let you place a deposit immediately before rental (imagine a rental car in a public lot) and then automatically receive your deposit back when you are done with the car (which was possibly even left in a different lot). I'm thinking the car would use its sensor system to detect if any damage might have occurred and if not, it could trigger the return of a deposit as soon as the vehicle is locked.
Admittedly, I don't really understand the distinction of a "smart contract". I am only describing the function of the system that there was some confusion over.
