It is not illegal to possess, however..............
It is classified as a research compound. So it is likely that there will be no recommended dosage or warning for children on the label
That means that the receipt of it would constitute a violation of Title 21 of the Federal Food and Drug Act since the compound would be considered "misbranded." Yes, I know that it is a petty thing, and serious nitpicking, but if "someone" really wanted to bone you, it is a misdemeanor punishable by up to a year in jail, and up to I believe a $1,000 fine. I could be wrong on the fine though.
I doubt, however, that any US Attorney would waste the time on prosecution of such a charge unless a great quantity was invloved. At the worst, if ordering for personal use, it would be confiscated, and you would get a visit from the postal inspectors and your local officer friendlies
Still, it would probably take a torn open or damaged package with the actual product visible in a postal facility in order for such a thing to happen.