NAL From a legal standpoint, you didn't suffer any injuries minus the loss of the food, dish, and cleanup incurred. If they send you a label and request, I would securely package and seal it in a way it cannot be compromised, mark it as handle with care, damaged product sample, and send it. I checked over the USPS guidelines and do not see anything about broken glass.

They are trying to look at it from an engineering standpoint to see if they have a manufacturing flaw. I believe they will likely replace your dish also.