Insurance is NEVER free from them. A lot of times if it's short term (not more than a few days to a week), your personal insurance will cover it, they'll just need to make a photocopy of your insurance card.

A lot of times, I've went on weekend trips, fly into the airport, wasn't asked about insurance, and find that once I've returned the car, they charged me for insurance without asking me first, not asking me if I had my own insurance that would cover it. It's taken several days to get them to refund the charge, so make sure you bring that up with them if they don't first.