My new climate control storage methods may also be a factor in our success so far?
The First Class shipping could also have some benefits beyond cheap cross country delivery.
I think tossing a 12 ounce package with less mass than a larger 2 pound box might result in less force transferred to the eggs due to less inertia when the package comes to an abrupt stop?
The biggest drawback to using First Class is that you need to fabricate two boxes of a fairly precise size.
I made blocks of wood that I use as jigs to pre-measure and bend the cardboard.