To be honest I am not a fan of Lipo bags. I would rather invest the money into ammo cans. You buy them fairly cheap and they can be vented if needed. You can charge in them if you wish. One thing I would suggest is rubberizing the inside and any sharp edges. Next: rarely does "mail" ship via commercial air liners. USPS, UPS, FedEx, DHL and whoever; all have their air liners. Not sure what a terrorist would gain by hijacking one of those. It would be like an episode of storage wars and even if the plane was hijacked whats the likely hood of the bad guy knowing that there are lipos on board.
Is there a risk with lipo batteries? Yes to some degree there is a risk. If your handling the pack in accordance with the manufactures suggested best practices your chances of failure are diminished. Now for storage, whats one thing fire needs to live? HEAT, what ever material you talk about it needs to reach a certain temperature in order to burn, flash point if you will. You could be the safest Lipo owner around with metal cans and everything, but you stored your packs in the attic in the summer time. For storage I personally keep them in the refrigerator. All I am trying to do is maintain a lower internal temp. This does two things, keeps the battery healthier for a longer time frame when not used and two keeps the internal temps below flash point. Is this fool proof, No but its better than nothing at all.