Sending powder, propellant via standard postal carrier is a really stupid idea if against the carriers terms and conditions, which it is for all but a very few specialists hazardous goods carriers.
It potentially puts at risk individuals in the delivery process and could result in further restrictions on ownership if some incident occurred, and likely cost the sender their firearms certificates.
standard powders are classed as 1.3c UN161 hence are classified as explosives their is no such “classed as propellant”
ownership of powders is licensed, but as a owner of an FAC, SGC, RFD then explosive regulations 2014 (ER2014) includes an exemption from requiring a licence to acquire and keep, (unlike black powder) a limited amount of net explosive quantity, which includes all powder in ammunition, cartridges or lose powder. Above the exemption limit you then need a licence.
Sellers of powder must see your certificate to purchase and record details as per ER2014
For details read the HSE explosive regulations.
ignorance of the law is no excuse in the eyes of the law.
edit to add, explosives are classified together with their packaging, typically cardboard box which will have a 4G code on the box, confirming the box has been tested as suitable to transport the explosive.
so it can only be transported in this original packaging.