Should be an interesting set of opinions. Not sure how it will fall out, though. The liberals will be pulled two ways, in favor of drug use, but also not wanting to weaken federal control of the economy. The conservatives will be torn between a dislike for drug usage, including gateway drugs, and a desire to give the states greater autonomy from the federal government.
My guess is that the federal law will be upheld as a valid exercise of the Commerce Clause given that all of the parties are clearly engaged in commercial activities, and purely intrastate commercial activity can be regulated under the Commerce Clause if it uses the instrumentalities of interstate commerce, such as the road network for deliveries.