I've ordered all of the eggs that I've hatched online and they were pretty small. None were broken and each one was wrapped in its own piece of newspaper. Have the box be packed with newspaper so that the eggs stay in place securely and write fragile on the box.
I haven't sent any eggs, but I have received a lot of small eggs (bantams and silkies). The eggs were individually wrapped in bubble wrap quite a few times, and then placed in the box which had been cushioned with shredded paper and/or bedding from the coop from whence they came.
(none were broken and the air cells all appeared fine, I just stink at hatching them)