@tree-hugger
You can use the shot cup, but should use an over powder wad regardless to ensure your BP load gets fully compressed.
BP has to be fully compressed, unlike smokeless powder, to function right and not be a bomb.
You'll probably find that using a shot cup with BP makes lousy patterns. Using a pre-measured charge like @vatopa described above works really well and the paper helps seal the bore.
As for the over shot card, they generally disintegrate and don't cause issues with the pattern.