Most places will NEVER take back live mice. Too much risk of illness being brought back.
Stress from being put into a snake cage, carted around from the store and put into a double stack of 5-gallon buckets is enough to kill a mouse. Plus.. what was in the buckets previously? What was it cleaned with? What air flow is there in it? Was the mouse dropped into the snake cage or the bucket?
It's not necasarily the mouse being defective, they CAN be delicate little animals, and any of the aboce could easily have killed it. I've had small rats die on the drive home from ???? stress? excess heat? heart attacks? alien probing? Who knows? Rodents are always dicey and can die from nearly anything.