Muggsmagee posted a fantastic link. You can leave unwashed eggs in the refrigerator for as long as 6 months. It's a great article to read!
I would really recommend that instead of an expiration date, you provide the date the egg was laid. It's really neat for customers to know that their egg was laid, say yesterday, and it gives them the chance to get educated about how long fresh eggs really last. Maybe you could provide a little piece of paper with information printed on it. Otherwise, they're never going to believe you that these eggs can sit in the fridge for 6 months.
Granted, after 6 months, they're not going to be "fresh," but they're not going to be "bad," either.