I would love to flush all the fluid out...but I won't let anyone else do it, so the only do it yourself way i've heard of is disconnecting the return cooling line, starting the car, and adding fresh fluid as fast as the old fluid is coming out...then once you start seeing fresh fluid, you pretty much flushed out all the old tranny fluid...but before doing this you are supposed to drop the pan and change the filter, then refill what fluid came out with fresh fluid...Then you start the cooling line flush thing...

So far i've only done pan drops, and filter changes...

If you have a shop do it, make damn sure they are using Dexron VI fluid...for $90 bucks, I doubt they are, even if they say they are...I would bet they are using Dexron III...I paid $55-60 just for the fluid alone last time I changed out just 7.5 Quarts worth...so 13 quarts would be close to 90 bucks...the dexron VI was OEM in your car, and is a much better fluid than Dexron III, but its also twice the cost of Dexron III...but you never want to put a fluid worse than OEM quality...