Please read carefully following pages where you learn about items, which are restricted from posting using Hong Kong Post.
http://www.hongkongpost.hk/filemanager/common/poguide/en/6.3.pdf
http://www.hongkongpost.hk/en/about_us/whats_new/notices/index_id_71.html
Please be noted, that ALL MAILS ARE CHECKED, so you better don't try post something what is not allowed. This will FOR SURE leads into full postage lost and it might even generate more expenses to get your package back to our warehouse (as sometime Avionics security might require collect blocked package in person).