I personally prefer to transport fish in the normal bags filled with oxygen whenever possible. If iti had been possible for me to deliver them personally, then they would have been packed like this in a large polystyrene box.
Because the fish were being sent via the postal system there are restrictions in both weight and size of the package. Also, the courier and postal system dont carry live fish.Therefore we needed to use the smallest and lightest size possible. Normal fish bags would have made the package too large and add the risk of the water being heard sloshing around when moved.
I hadn’t considered small plastic screw top containers, but they may be an option in the future.
As I said, they are not my preferred way to transport fish. But they are an option if space and weight is a problem.