MaineDog, I can't see any reason why anything should happen to the bellows whether traveling pressurised or otherwise. Maybe just wishful thinking on your part??? Naughty thoughts!

I would recommend that you ensure that the bellows straps are secure, but I'd be far more concerned about cargo handlers and controlled explosions than about the bellows bursting.

There is some evidence that extreme pressure changes could cause fractures to the wax holding the reed plates in place, but as some one who has regularly shipped and received accordions by air freight, I have never experienced this phenomenon.

MC