My 2 cents..
Try not to worry too much yet. Please let us know what the vet finds. There are so many ways chickens get sick. In the transport boxes, there are biological filters over the holes. These are just to help keep airborne bacteria from coming out of the box while in transit. Shipping does stress and makes them more prone to things they have already been exposed to and also all the pathogens of a new place that they may not have any immunity to. Keep in mind that NPIP gives a certain amount of false security. It's only a yearly test for 3 types of salmonella including pullorum and 3 types of Mycoplasma and now avian influenza. They could still have lots of other diseases going on as well as things like coccidia or parasites....
Not trying to make things sound worse...Just saying, see what the vet says, keep the seller in the loop, and use some quarantine practice until things are normal...