Forget the flush, just do a fluid and filter swap. There is DIY write up on how to do it. I bought the fitting off amazon for under $10 and the filter was not much more. Its quite simple to drop the trans pan and change the filter. Using a handpump to pump back in the 5 or so quarts of fluid was the only difficult part. Only because your on your back with a crappy handpump.....it takes time and your arms will cramp up. Adding a transpan with a dipstick will solve that completely. You then add the fluid via the dipstick. You can buy a deeper pan or the same size stock one with a dipstick......very nice mod.
Search online and you can find these items cheaper.