Driving on extremely low transmission fluid

[pmcevoy]

I drive a 2002 Mazda MPV which has been a very dependable vehicle. It has 109,000 miles on it. While visiting family, I went to a chain place for an oil change and transmission fluid flush and fill about 50 miles from home. Part way home I noticed a whine. Took it to a mechanic who showed me that the transmission fluid didn’t show up on the dip stick and there was no leak. I called the chain place, talked to the manager who seemed properly nervous. He instructed me to have the mechanic fill the transmission fluid to proper levels and return to their site asap to deal with this. It took 3 3/4 quarts to fill. The transmission seems to be shifting smoothly at this point (driven 15 miles since fill). My question is this: How much damage was likely done to the transmission given approx. 70 miles on extremely low fluid? I don’t know how to “settle” this with the chain store. If it had “blown up” the case would have been clear. Now problems may not show up for a while. How would you handle this?