I have read about broken bottles but it has never happened to me. I have had successful shipping of glass bottles from The Plume Room, Halo, House Of Liquid, Vermillion River and Ahlusion. If shipping a "poison," extreme caution should to be taken irrespective of the bottle material. Glass can break, platics can be punctured or compressed and subsequently opened. All glass bottles that I have recieved were shipped immaculately--the bottles were shipped as if they were, well, glass. As a side note, I have received many plastic bottles that opened, from one cause or another, several times.
Yeah, all of the glass bottles I've gotten from Want2Vape and Halo, as well as the empties I've ordered from elsewhere are really thick glass that would require considerable force and abuse (not saying USPS isn't capable of that) to break. I don't think it would take as much to mash a plastic bottle, leaving you with a bag o' juice.
Nicotine is still a poison in large doses and many places require childproof containers etc.
Maybe if childproof caps are required in some places, those vendors could have an option to include some of the non-childproof variety with the order that would work on the same bottle. Free would be great, but I'd pay extra at this point, within reason.