Purchasing, possessing, or selling clen in the US is the same as doing it for any other controlled substance. The biggest thing about clen is, alot of it looks like sutifed... so it's HARD to know exactly what it is.
It is very easy to confuse with headache medication or cold formula. However, ordering from an online site is not the smartest thing in the world. First, many of these sites scam. Secondly, many of them are flagged by customs, so no matter WHAT they send you, they are going to be seized (or at minimal, opened and searched). Finally, some of these such sites are set up by the government to trap people into purchasing illegal merchandise (no, it is not entrapment to do something that one is already predisposed to do).
Basically... Clen is a controlled substance... you used to be able to buy it in GNC... believe it or not (GHB for that matter!)... but now, it is controlled. If you are caught with it (and the agent/officer knows what it is).. it will be confiscated and you will be charged with possession and possibly for the intent to distribute.
Keep that in mind.
C-ditty