It is not allowed to send explosive substances (such as black powder, nitroglycerin, ...) and pyrotechnic articles (fireworks) by mail because they are classified as hazardous goods; their transport and shipment is subject to specific provisions under hazardous goods legislation.
Should you intend to ship such articles you are strongly recommended to inquire about the relevant provisions at the post office or the carrier assigned for the transport.
In any case of shipment by post or online commerce, our guidelines summarised in the section "Movement of goods within the EU" apply by analogy.