Normally insurance agencies only offer insurance on a rental IF you were given a rental because your car is in the shop. I don't know if they give it to you on a case basis for renting.
If I have to get insurance on a rental, I get the loss and damage waiver. On average it is $12 a day and then you can beat the hell out of the car. Take it off-roading, throw rocks at the windshield, get into a gunfight during a high speed chase. It's all good! I'm exaggerating but if you get a ding or a rock hits your windshield or you get some other normal damage, or the car is stolen, you don't have to pay for anything. I had a rental company in Austria try to charge me for paint transfer on the side view mirror but thanks to having the waiver, the $250 they wanted never saw my pocketbook. Another time I rented a car in Chicago and went to Canada and blew a tire there. Not wanting to drive all the way back on a donut, I called them and thanks to the waiver, they had another car sent out to me. Of course it was fun to think someone in Chicago would have to rent a car that had km/hr on the speedometer rather than mph...
"Also, I would prefer a back seater over the extra gas any day. I would have 80 pounds of flesh to eat and a pair of glasses to start a fire." --F/A-18 Hornet pilot