Good Morning and Great News that they made it...! as an Express Carrier for the USPS, I want to let you know that as frustrating and wrong that it was to let the parcel go astray...
most postal workers and contract drivers really do take extra care with live animals... and I'm sure the folks you talked to, while not able to locate them for you, were apologetic (I hope!) most "hub" locations will do their best to maintain the animals til the next departure if they've gone astray. Many actually have heat lamps for the very young chicks, and will provide water at the very least. I have a friend who is a pilot for USPS. he "lost" a rooster (left it on the plane...

!) and it cost him $12,000, you know he will never be so careless again!
not all areas have Sunday and Holiday express delivery anymore, due to the financial crisis of the USPS, but if you have an Express with live animals, you are more than welcome to go to the GMF (general mail facility) in your area and pick them up as soon as they arrive ~ if you make sure you put phone numbers on the shipping label, the supervisors at the Express Expediters
will call you...
In Albany, NY, we still run Sunday and Holiday Express routes, I love chick season... I wear layers, so in March when they start shipping, I can wear a t-shirt in the car so I can keep the heat blasting! Folks are always so pleased to receive a toasty warm box of chicks...
but... mistakes do happen. that is how I got my chickens. on my normal Express shift, a box was discovered in the cart bound for another state, and all the trucks were gone for the day. they were 2 days old and would not all make it another day... I had a box of 200 chicks for an aggie store, and offered the waylaid box of chicks to them for free and they said "no". I thought about slipping a chick or two into each of the other cartons I had... but these were all dark chicks and all the rest of my shipments had yellow chicks... didn't think it would go over all that well, so I kept them. I divvied them up and shared with my good friends. they all made it

(my supervisor had already contacted the hatchery and the customer, it was decided that she would get a new shipment and we would do our best to not let them expire)
I'm so relieved that your story has a happy ending!