For those who think no one is at the post office on holidays, think again.
I worked for several post offices and had to be there on Sundays and holidays to receive mail and to also deliver the overnight packages to the addressee by noon. I was the only one there and spent half my time delivering the overnights to very rural addresses so I wasn't always in the building. These were small rinky-dink post offices, not the large processing centers.
I know that when we got a box of chicks in we called that person ASAP but unless they paid for overnight they had to come pick up the chicks. I would tell them the time I'd be in the building.
However, that said, I agree that the post office could have been at fault.