Go easy on the USPS everyone, they are the ONLY carrier who will deliver chicks (or bees!). UPS & FedEx won't handle them last time I checked. My dh is a carrier and they all are very concerned about making sure that chicks get where they're going safely but many don't realize how warm they need to be. He had to explain to his office that putting them outside where it's 'cooler' isn't the right thing to do (like it would be for a puppy or kitten), they may very well have thought they were doing what was best for the babies.
I'm sorry to hear about yours, it might be worth going to the PO and explaining how warm they need to be before any replacements are on order.