There usually are a bunch of taxes and assorted minor fees that get added on. These aren't optional.
In addition to the collision damage waiver (it's not really insurance, per se) that you should decline unless you have no coverage through your own insurance or your credit card, the main thing they'll try to sell is a fuel option. With that, you essentially buy an entire tank of gas from them and don't have to pay their exorbitant refueling charge if you bring it back with less than a full tank. If you're going to drive a lot and could go through a full tank or most of a tank, check out their price per gallon. It's not always a horrible deal, but bring it back as empty as possible. If you're going to use less than a full tank, you're better off refueling on your own and bringing it back full.