Even if it's just in transit from one "doper state" to another?
Note that I said Sane state. However I don't know at the oment howthe fedgov could provide a secure border to the sane state while allowing the same material to be transported to a doper state. I've not spent much time thinking on this yet. The key point is if a state wants to keep it out they should be able to and the fedgov should help.
If as state wants to ban posession of a particular item, say whiskey, or hand grenades, or marijuana, they have the authority to do that. The original intent of the Commerce Clause is not for the federal government to assist the states in interfering with commerce that passes through their state, but to prevent them from doing it. Under what enumerated power do you submit the federal govenment should assist the state in preventing it?