Very sorry about all the losses in your tank.
So true, I have had losses of about 60 Neons over the past month and a half. Sick fish and mine were in QT so that was an advantange I had but sad and maddening.
Any fish can become stressed and ill during shipping and handling even if they left the breeding /shipping facility in good health. They may not have been healthy either when they left. Can't know at this point.
My last batch there were quite a number of dead fish from 24 hours at the store but feeling like I could give them a good environment I was willing to try to and get them in condition to live in a beautiful large school. That was a bad call. Does make me very sad to see.
I understand what you have experienced, by having the same thing you have happen and it was from a reputable lfs
and what made me madder was that later when I called and talked to them, they mentioned they had had a bunch of then die and sold me the last 3 fish anyhow!
So it can be any situation. Private owner store or big box store. After that time I started to do QT.
Again very sorry you have had this happen. You are not alone.