I would also mention that cardinals are notoriously sensitive fish and that even healthy batches don't always ship well. When I worked at a LFS I always sourced my cardinals and neons from local suppliers as they always held up better. I also think that its hard to judge any supplier by the health of such a sensitive fish. He could have sent you the best batch of these little guys the world has ever seen, but if the delivery service mishandles the shipment (and they frequently do), then you could end up loosing half the batch and the seller would still be at fault.
Just my two cents