Send them to customer service. You can use Zazzle's standard reply (the link at the bottom of the chat window where it says "Does this person need customer service? Send Auto Response.) Or you can write your own.
Here's my stock reply:
Hi _____, thanks so much for choosing my design! I'm afraid I'm not the right person to contact about that. I'm the graphic designer who made the template that you used, but Zazzle handles all orders, printing, and shipping so you'll need to contact Customer Support for help with that. You can reach them via www.zazzle.com/about/ask or choose "Customer Care" where you found the link to me (see attached image.) Thanks again for choosing my design, and I hope you get it sorted out!
