Yeah, the apple cider vinegar is a good idea.
I'm not sure what's doing this. Are they pooping? I wouldn't recomend the corn and oats at this age, but you did say you've used it before. I always felt that would stop them up when they're so little.
But I do stuff other people don't. I didn't give any of my chicks grit, and they didn't have access to dirt until they were 2 months old. One batch is only 6 weeks, so they haven't had dirt yet. But none have died, out of 46 chicks, and some people might think I took a risk or something.
Are they huddled together? Or spread out and panting?
You've had chicks before, so I'm guessing you know what to watch for on stuff. I'm baffled.
It's great that McMurray is sending you more. I'd suggest not putting them in the same spot, just incase it was something in the garage or outside air. Who knows. Maybe someone near you sprayed pesticides and it blew in. ???? And I wouldn't put the new ones with this batch (if any survive), until you are sure the first batch is ok.
Wish we could figure it out. But maybe it was something completely out of your control....... severe heat while traveling? chemicals at the post office? or could the food you bought be bad in some way - spoiled? Without knowing what caused it, you might want to start fresh...... new bag of food, different location, and just double check you've got all the conditions good for water/heat/food. This just doesn't happen often though. Chicks are fairly tough, and can handle variations from absolute perfection. I raised mine in an outside brooder, with a 100 watt bulb, sugar water for only a couple days, no grit, 30% protein medicated game bird feed (added cracked corn after 4 weeks), on newpaper - dipped the beaks and sprinkled the food. That's about it. Other people have variations, and some say never do this or that (some of the things I did).