********** on deposit with 'insurance' charge never explained

What a terrible experience with Hertz. I booked and paid for my car ahead of my trip. When i picked it up I was asked to swipe my card for a deposit which was a held transaction against my card. Several days after returning the vehicle they refunded only half of the deposit. When I queried it with customer service their response was that the charge was for 'insurance'. I was never advised of this charge and the amount was almost as much as 8 days car hire???!!!! What a total *******. Would not recommend them to anyone.