A 3500 tab does not mean you’re dealing with a sleazy mechanic. If they quote you 2500 and then try to nail you with 3500 then you have a valid argument.
I’ve installed a lot of add-on A/C units (never on a Kia though) and I see no way in the world it could be done for a 1000 dollars; and it’s been a long time since I’ve installed a unit.
Unfortunately, you waded into this and will have to pay the consequences. You live in the South knowing full well what the heat and humidity is like and will have to pony up or suffer.
Temps in the 90s sound good to me. It just hit noon here and it’s already 102 here in OK.