You mean the bulb like in some Serge Lutens bell-shaped bottles?
I used to send a lot of such bottles and nothing leaked. My advice is (I hope I'll explain it clearly, my english is not so good): I take some piece of a bubble foil (the kind of foil that it's good to wrap bottles in), I put this piece of foil on the bulb like I was going to hide the upper part of the bottle under the foil. Then I wrap the foil lower and around the bulb with a rope or adhesive tape. There's no chance that the bulb falls out, the foil keeps it tightly in the right place.
Hope it was clear enough