I would just go to a auto parts store and get some scratch fix or black touch up paint. Perhaps when you return it they won't notice.
That's what I was thinking. I'm guessing the rental company would charge quite a lot.
I think the real villain here is the person who scratched it and drove off.
I had a quick look through my rental agreement and I think the bit that covers this is "Collision Damage Waiver". There doesn't appear to be an excess so I think we are fully covered. It may require a bit of an argument with them when we return the car and keeping a close eye on our credit card charges just in case.
That's fraud, honestly. You damaged the vehicle (or it was damaged while in your possession). I doubt you would accept a haphazard repair on your vehicle if someone was borrowing it from you.
It's also fraud when the rental agency charges the customer for a "real" repair but only does a quick touch up, particularly if the rental agency then charges another renter for the scratch.