Hi. We were sent 100 chicks from a hatchery in May. Apparently that hatchery will only ship chicks in batches of 100 for increased safety (we shared them with several households). They were shipped in the morning and arrived in the afternoon, arriving in a box divided into four sections, each section containing abou 25 chicks. Some seemed older than a day old, probably a couple of days old. One chick did die before it reached us (smothered), but all the others were ok though stressed and we quickly went to get them settled. We have had no losses since then, ie. a total 99% survival rate (100% survival rate of the ones that reached us). And they have been happy and healthy. So your situation seems quite a contrast. To me that seems like way too long for the chicks to be in the mail: it is stressful and dangerous for them, they can only last so long and need reasonable conditions. If my chicks were as stressed as they were after around nine hours of travel, your thirty- six plus? hours of travel seems way too long. Then it is also possible they could have gotten overheated in travel at this time of year. Or 12 chicks is not many for keeping eachother warm, which is also important. I think they just may have had too bad a travel experience for too long.
Feeding them with a dropper may run the risk of getting water accidentally into the wind pipe which apparently can kill them, so do be careful with that. Handling can be a big stress for them too. I'm so sorry for your experience. I would think that to try to keep their environment now as calm and ideal as possible, a good temperature (around 95 degrees from a red heat lamp), and leave them alone enough that they have the chance to really calm down, always with the fresh water (apparently best with electrolytes, for stressed chicks) and fresh quality starter mash. Just thoughts. Also chicks get very stressed if they are just one so if you get down to one left see if you can find another chick somewhere somehow?
Afterthought: I wonder if Bach Flower Remedy "Rescue Remedy" would help the chicks (a drop in their water bowl). It can help traumatized or stressed animals and people.