Sorry for the losses. The beginning is bumpy for sure, so don't be too hard on yourself. I don't know your LFS, but I know mine not have allowed you to take home a fish like that to acclimate. At a minimum, he would have provided a lot of warning about the risks of a shipped fish -- especially if it were transshipped. Transshipped fish can spend DAYS in a bag. Once the bag is opened, the ammonia must be neutralized. The salinity can also be SUPER low relative to ours.
Next time, let them acclimate the fish in store and keep it for a week or more. You come in and watch it eat. Then take it home. If they are unwilling, I would be looking for another LFS, personally. Also, ask about their salinity compared to yours to understand how fast/slow you should drip the fish.