Hi Wendy! I guess it depends on where you are, how well the eggs are insulated in their wrapping/packing box, how well you trust usps to handle a box marked PERISHABLE as perishable, how often you can collect eggs in freezing (or near 'bout) temperatures, and how much chick down/dust you are willing to put up with all winter.
I hatch here year-round.
I've been wondering that myself(shipping)...I would say between 40-80 sounds right. I hatch all year if the girls lay, but it depends on if you have a warm brooder area outside or inside? I have a good area in my basement for peeps/incubators.