I had the same issue with every school of corys I've bought from local LFS, half would die within days or on the way home. This was even after a 2+ hour slow drip acclimation.I agree with some of them not being as hardy as others. Seems like half of mine always die within two weeks. Sort of spendy little guys too
I took a chance with an out of town breeder who taught me that they release a toxin when stressed. She described to me what she called the "kick the bucket" method she used before bagging and transporting/shipping. She would place them in a bucket, lightly kick it to stress them, change all the water after 15 minutes, and repeat this whole process once more before before bagging them. All 12 corys bought that day survived and are still alive now 6 months later.
I know that doesn't answer the OP's question, but just an FYI in case anyone (like me) didn't know about the "cory toxin" lol.
Interesting read on it: Self-Poisoning: A Damaging Defense Mechanism | RateMyFishTank.com