It just sounds really fishy and I'd hate to see you out time and $. He didn't purchase insurance so that is his fault, you wrapped well, any money he owed for postage is wrong and why is he so nasty? Just take some photos and be nice about it.
I got eggs a month back with 3 of 8 completely...
Have you shipped new eggs yet? I wouldn't, not until you have proof of the extra postage owed AND photos. What's to say he still won't leave negative feedback once you ship free replacements?...yep I have actually had people on ebay leave me neg feedback ( not on eggs ) after I resent...
How can you owe postage on a flat rate box? Sounds like he just wants free eggs. I received broken eggs and was happy to send photos to the ebay seller.
Unfortunately there are crooks out there who will use negative feedback as a reason to get free stuff. What is HIS feedback like?