We booked one in France through Avis. Rocked up and they wanted either a €2000 refundable deposit or a €350 non-refundable waiver. We were deep into a home renovation, couldn't afford the refundable option if we also wanted food on holiday, had to take the hit.
That's over the top of a €950 10 day rental for a 3 year old Seat, its gearbox nearing the end of its short cruel life.
And we were treated like this because we'd used a reseller. The person before us didn't have our experience and the guy there was happy enough to explain how they hide details like this from comparison websites and everyone assumes they'll get the standard rental agreement. Loads of people get stung on the waiver.