Solved:

I figured it out when I played around booking a new reservation and I couldn't unclick it there either. That click in the summary page changed the settings in my profile. I changed the setting back in my profile (I also had to log out and then in) and it was fixed. I just edited the reservation in question.

This is good for my trip bc I would have brought that car in completely empty if I had been stuck with fuel option bc prices stayed higher. I calculated that I'd drive enough that trip to need a refill but bc I was forced to take it, I know if purposely drive extra the last day just to burn more gas for return. It's.better for the environment and for my sanity that I was able to fix it.

Interestingly, I looked at my last fill-up for my previous rental and it was $46 so I might save a few $ taking prepay but I know I'd just waste time trying to minimize my fuel on return. Like I know I'd keep putting 1-2 gallons in when I got to empty near the end of the trip in order to maximize prepay and my GF would get annoyed.