I already made my position clear. I would not have paid if the original estimate said $0. Period, end of story.
Knowing the car takes 6.5 gallons, and if it is warning you it is empty, it is an impossibility that 2.5 gallons would 'refill' it and the system be OK. There are two possibilites: they didn't fully fill it or the warning is wrong.
(Also note that parts sells one jug that is 2.5 gallons. If they put it all in or put in 0.5 gallons you pay for the jug.)
Between being ignorant and lying, most SAs are useless.