I never, ever, for any reason leave keys in a box. Period. Somebody dropped them, lost them, stole them, their kid kept them as a toy. It's a he said-she said and you lose. You are lucky it's only $150.

In those rare occasions when the box is the only alternative, I call in to say I forgot to drop them, I'm Fedexing them and they get the keys the next day.

Short of finding out if there's a video of the counter and you can get ahold of it (which will cost you a lot more than $150 in time and outlay), there really isn't much to do here.

You can always dispute the charge, but car rental companies are famed for their documentation and you will just lose the dispute down the road.