Corys are one fish that i will never acquire if they have just come in to the store. I always wait at least a week. It doesn't happen always with all stores, but I have known very reputable local stores have up to half their corys dead within 2-3 days. I'd rather the fish died in the store tank than in mine.
One reason for this is improper shipment of Corydoras. This fish does not tolerate ammonia. They must be shipped in very large bags/boxes, or few fish to a container. As TheWunderfulWub said from his own experience, the trauma fish go through from capture/farm, to distributor, to store is significant; I often wonder how any fish even make it.
You mention Corydoras julii
, if you have the true C. julii species, it will be wild caught; this species is quite rare. Most corys labeled as julii are in fact C. trilineatus. This species is hardier, but as far as I know is still wild caught.
You should be able to get a credit, but please wait a week before selecting any more. If they are alive and acting OK then, you will probably be fine. I have stores that will hold fish for me, for this reason; they too would rather the fish did not die in the bag to my aquarium.