That is , the first ones after the cotyledons, the first two funny looking ones out of the seed. When shipping one has the balance between 'is it big enough to survive' and 'is it too big to ship economically' to consider. If you pop the tiny seedlings in some damp paper towels, put them in a ziploc bag, and pack carefully in a box with some crumpled dry newspaper, you should be safe.
Heat should be considered. Where are you shipping? What does the person you are shipping to know about their mailbox/mailperson/ mail van? Have they gotten things alive or crispy dead in the past?
Citrus is illegal to ship to Texas. I suspect other plants have restrictions as well, I think California has some interesting regulations. This sort of thing would be best done on an Internet search by the state you are shipping to.

you can also call your local state/ city agriculture/horticulture division and they can tell you over the phone or send you a brochure. Sometime your local state colleges know as well. I also got help from my in town Botanical conservatory and state park centers. In Columbus, Ohio you cannot grow Morning glories on some parts of your property because they are considered a noxious weed if they peek thru fences onto city property.
Seeds can also sometimes be a problem depending on where you live in a state.