If using pre-weighed charges then I transport them in plastic vials in a plastic .44 magnum MTM case. If planning to use a flask at the range, I carry the powder in it's original container for transfer to a powder flask at the range. I prefer not to transport BP in a flask as I find it leaks somewhat, and I am also concerned about absorption of water vapour - I guess putting the flask in a plastic ziplock would solve both concerns.
A member of my club travelled to the Vesuvit importer (somewhere North - about 200 miles each way) to procure 32x1kg plastic tubs of their finest as a bulk purchase for the club. He became concerned about transportation so he asked the importer who informed him that he could carry up to 50Kg in his car without any extra paperwork or precautions . They also supply it in a 16kg plastic bag in a cardboard box (slightly cheaper but a faff to divide up later) and this is also OK up to 50kg. Surprised the hell out of me but it is legit.
IIRC about 1lb of BP will self confine (transition from "woof" to "boom") if ignited in a loose pile, so I am sure that a 16kg bag will .