hmm I'm not sure I'm not nearly as experienced as some people on here.
Try asking questions in a popular thread that already has lots of people responding.
Have you seen them all drinking water? And have they regular chick crumb? Have they got a nice warm area, but can easily get away if they get too hot? Do they cheap a lot?
It might be the stress of transit, shipping live animals in the post is illegal in the UK I know that for sure. There must've been a few weak ones, either through luck or their gene pool. Sorry to say it, but maybe the first couple days thins out the weaker ones.
Once I got 60 young goldfish off this lady for free because they were filling in their pond, after deciding it was too dangerous to have with 2 small children, and I was so pleased I got soo many for free. When I got them home I gradually introduced them to the new pond, everyone seemed happy. After a couple days I noticed some were acting slow and not eating as much, not seeming as active and having trouble with their buoyancy, within a couple months, I'd lost nearly half the goldfish, very there would be at LEAST one dead and one other starting go strange, sometimes 3 in one day. I thought it would never end! In the end, I ended up with 5 GOLDFISH and a couple young black ones I could barely see. There must've been an illness between them, bacteria levels changing in the season and the stress of transit.
But since the last weak one died, no one has died and all look incredibly healthy, even when it's freezing outside.