First, I'm sorry you lost your chicks. Under normal circumstances, I don't think that would happen. It sounds like they are being forced to ship on Saturdays, probably because that is when their local post office can handle their volume. Their response, to me, sounds pretty reasonable. The mail doesn't stop moving on Sunday. The birds are still in route to you and most often probably would arrive on Monday. It sounds like that simply didn't happen in your case, but a shipment getting delayed like that can happen regardless of the day it is shipped. I don't know that them being shipping on Saturday had that much to do with the loss.
As far as Express verses Priority, it is a complete and utter waste of money for us to use Express here where we are, YMMV. It *never* get here faster than Priority. In some places, Express is now taking LONGER than Priority.