It is true, I am told by Cory shippers, that Corys or some Corys do sometimes release a toxin that can kill them if they are in a bag. But usually when shipping, after the fish is caught it goes into a holding container, then to the bag when it is ready. This process of catching, releasing to holding, and recatching to put in the bags does generally take care of that problem if any Cory is inclined to be suicidal.
I have recieved hundreds of Corys from a variety of shippers. Usually they do fine. But occasionally they are not well prepared, bagged and packaged. If there is a delay and they have not been prepared well, one dies, and then the whole bag can go quickly. I have recieved my share of horrors. Often many who do arrive alive after that will not survive. I am inclined to think that the pandas you recieved were of the sensitive kind. Some more sensitive fish will not acclimate well to the conditions. If you buy from a breeder with soft water and a low PH and your conditions are hard water with a high ph and the fish are sensitive, no amount of acclimating will do much good. It takes weeks and months for fish to acclimate to some new conditions. If the fish are solid, they will acclimate.
I do have about 15 panda juvies. They are about 5 months old or more. They are F1/F2 from the fish I mentioned before. If you are possibly interested in some, PM me. It is not possible for me to tell their gender yet. If they were peppers or bronze they would already be tying to breed, but pandas seem to develop more slowly. I shipped LF albino peppers to ICEEGRL and they did well, although two died much later: a very young fry failed to thrive and eventually died and an adolescent died later of unknown causes. I had sent 2 young fry extra with the package, so she has the five she wanted. She has been busy trying to get viable fry from them.