Surely that is the vendor's responsibility.I would have a go at the couriers, and if you get more fish by courier, make sure the box has "LIVE FISH" "URGENT" "OVERNIGHT DELIVERY" "DO NOT LEAVE IN HOLDING AREA" written on the box. Then make sure the courier company can deliver it overnight. If they can't guarantee overnight delivery, don't get the fish or make the courier company pay for the dead fish.
In the UK buying 4 corys online would not be economically viable because shipping would cost more than the fish (at least from a reputable supplier). But they always pick up and deliver exactly when they say - and packing is not the courier's responsibility.