It's safe to ship them the moment they hatch. However, that's impractical since the recepient will not be able to see them all in the bag and inadvertantly will throw some of them into the garbage with the bag.
A good rule of thumb is...ship them if they are easily visible to the naked eye.
How do you do water changes without sucking up the babies if u can barely see them. Now im thinking I might of have killed hundred of babies without ever knowing it.