[Dixie]

I believe you need to respond in order to mitigate any loss of your current, and future, customer base. I'd do this by responding in some heartfelt, yet non specific manner. Something like, "We at xxx printing are sorry to hear of your unfortunate experience. Rest assured, xxx printing is currently investigating this matter. Once completed, our company will address this incident in an appropriate manner."

Notice there's no admission the incident happened, who's at fault, or what exactly will be done.

PS: I'd make it a new store policy to ask if emails can be opened. If the answer is no, kindly inform the customer you can't complete the service, and show them the door.