I'm sorry this happened to you! I would suggest that you let them replace the chicks or get a refund and order from another hatchery. We got our Australorps from McMurray and they are all healthy, but I guess this doesn't always happen.
Maybe the chicks got too hot in transit or something. I bought 6 chicks at a flea market Saturday that got too hot in a plastic box the stupid lady had them in. (I've already been blessed out for buying them from her, so please don't preach at me.) After they started dying, a friend gave me some stuff to put in their water with electrolytes and vitamins in it. It perked the others right up and they seem healthy and happy now. Maybe your surviving chicks would benefit from something like this?