Wait
Do not do the flush unless you know when the last flush was.
You risk moving around too many metal shavings and that could AND OFTEN DOES DESTROY THE TRANNY!!! Look at other posts about any trans for any car, you should never change the fluid if its old, burnt smelling, AND you don't know when the last change was.
If you want to do anything get some LUCAS tranny stabilizer fluid for around 10 bucks and put that in IF THE FLUID IS IN OR BELOW THE CROSSHATCHES on the dipstick. If the fluid is almost to the top of the crosshatches you CANNOT add more it will damage the tranny.
Look around for another trans shop too cuz any tranny specialist would know that if you dont know when the last fluid change was you just add fluid or risk ruining the trans. so this shop wants to ruin your trans and then charge you to put a new one in at around 2000 bucks to do so.
The only way you are okay to change that fluid is to have a REPUTABLE shop check it and if you have under 100,000 miles and you think the fluid was changed at least once.
If you get it changed anyways you may be okay but if its gonna go because of the change it should happern within 500 to 1000 miles after the change.
What happens is the trans has valves in it that MUST be clear of small metal shavings and stuff. If the fluid is really bad and the tranny is worn badly too, there WILL be metal in there and if you do a full flush OR T-Tech it will move those around and likely clog a valve/ vein of the tranny and this is bad.
Good Luck and I hope you get this in time!