I think all rental companies have the "approx cost" thing, protects them in case there is an increase in taxes or fees.
Generally speaking you will pay the total as listed on your reservation.
I would imagine that if Florida raised their state tax .5% between the time you booked and when you actually rented, you would probably be expected to pay the extra .5% taxes.
The only time I ever saw a difference on any of our last 30-40 rentals was when I added my New Southwest number to our reservation so I could collect airline points, there was a $0.74 surcharge added because of this.
FYI: Hertz does not allow you to pick your vehicle.
That being said, I usually just ask "what they have for me today" and I'm usually given 2 or 3 options to choose from.
Hertz does have a skip the counter kind of thing if you sign up for their Hertz Gold Membership (free to join).
Checkin online and then go out to the booth in the garage to sign and get your keys. The line is really short (last couple of times, we were the only ones there).
Plus side is that they will send you a text or email prior to your arrival to tell you which vehicle you have and what stall it is parked in.
Downside is that you don't get to pick your vehicle, although I'm sure if you didn't like what was assigned, you could ask for something different (it just adds more time until you can get out into the Florida sun).
****I did a last minute "pay now" rental with Hertz and they did a weird split in the payment, I was charged about 1/3 of the price immediately and the remainder when I picked up the vehicle. Not sure why they split it up like that. This was a couple of winters ago in Phoenix so your experience may be different this year and at MCO.