You would be better off moving this into the Universal category - use the little pencil next to the title.
As far as I know, there is no discount on 1 day tickets but I’m in the UK and others would know better than me.
Universal doesn’t have FPs like Disney. They have Express Pass, which you can buy (on top of your ticket) or get for free if you stay in one of their deluxe hotels, and puts you in a shorter line like a FP, but for everything (except the new Hagrid ride at the moment). The paid version is either once per ride, or unlimited. You don’t need to book anything in advance - you don’t even need to buy it in advance, you can add it on the day if you want to. I don’t think it’s worth paying for personally, but many disagree with me. And if you want to do as much as possible in both parks, not just the Harry Potter areas, in just one day then you do probably need it.