http://bazaarboom567hsuxjspmwurpl7lyx23p7r2byg22vwfhv5yubvvezid.onion/faq/for-vendors/dead-drops-guide
A magnet is put under the outer layer of the packaging and allowed to securely attach a drop in a most unusual places like on stair railings, on road barriers, on traffic signs, and other metal structures, which are spread commonly in every city.