Shipping black powder is difficult, especially by air, because it is considered an explosive... One way someone I know is using to transport powder by air is to load it into large powder capacity cartridges! Shipping or transporting ammunition is easy and legal... it takes about 25 cartriges of .50BMG to ship a pound of powder for exemple... I believe you could use 12ga blanks (with a used primer to avoid accidental discharges in a gun!) and have that “ammo” shipped to you... Then remove the black powder and store it an empty powder can you kept...