You can blame the dogs all you want... what if it was something that could be damaged by water and was left out in the rain? Would that be the rain's fault or the person who left it open to the elements? It really is the same thing.
How horrible! I'm so sorry.
I would make them replace the chicks-they have a responsibility to deliver things safely. What if it had been perishable goods (worse even that it was live animals). I would also take to the postmaster and higher up, as well as place a complaint through the...