no need to pay them anything extra. Before you turn down rental car insurance, check with your insurance company, to see if you are covered under rental cars. It wouldn't hurt to check anyway in case you have to answer that question in the future. That way you will have the information in case you need it.
If your insurance doesn't cover it there are other options. If you don't own a car you can get a better rate through an insurance company that offers customers who rent cars frequently. This will be a lot cheaper that what the rental place will charge you. People who don't own cars can get a bundle for driving rentals. Paying for this can get you a price a lot cheaper. This gives you the security of knowing you always covered.
If you find out that you need non owner insurance, foresight and panning will save you a lot at the rental desk. Temporary car permanent savings.