I think with bought eggs, there are too many variables to blame the genetics on weak hatchability. The post that said that later they cannot tell which ones were late or weak hatchers shows that weak hatchers can thrive too.
With shipped or purchased eggs, maybe the weak hatchers are the older eggs, or ones that got a little more rough treatment and have less stable internal structure. The genes of the chick may be just as good or better than the strong hatchers.
So IMO if they are not your own eggs, I would help any chick that has already zipped all the way round and sits for more than a few hours. It's probably stuck.
Roger
Definitely i agree with that, shipped and home eggs are different.