didn't see any signs of injury, had some ad mites and bathed him for it, but he kind of started hiding in a corner and got listless
Mites are horrible on birds, Mites + shipping stress is disasterous. Mites are blood suckers and the stress they put on the body and sucking them dry on top of a compromised system from the shipping stress sounds like that did him in. Or it could have been Cocci on top of that. Did he ever touch the ground while you had him? If he went outside on dirt/ground thats my bet - if he was indoors the entire time I'm just going with the dual stressers.
Only thing I was told and have done with shipped birds of any age is keep them quiet the first day give them a booster (vitamin/electrolyte water, sugar water, whatever something to help give them a boost) then feed them some boiled eggs and crumbled food. When I take them out of the box at home I look them over really well for 2 reasons - 1. to look to see the bird I got was what I paid for, 2. injuries, mites, signs of sickness, etc. Then after all that I slowly start treating them like any other bird but keep the quarantined.