Leftover water is the enemy here. Empty and dry every part of a bottle sterilizer before it goes into a sealed box, or you risk leaks and mold in transit.
Pro tip: Run a plain water cycle after the last use and before packing to flush any remaining residue from the heating chamber.
Watch out: Leftover water in the reservoir can leak into other packed items or, in cold transit conditions, potentially damage internal components if it freezes and expands.