In California necropsies are free, but the body should be refrigerated, not frozen. You could talk to the vet. I would want to find out the cause if possible. Here is a link about sending bodies for necropsies in CA:
https://www.backyardchickens.com/t/799747/how-to-send-a-bird-for-a-necropsy-pictures
https://www.backyardchickens.com/t/799747/how-to-send-a-bird-for-a-necropsy-pictures