In some places a single seed will get you manufacturing with intent to distribute. If that is combined with mandatory minimum sentencing it becomes something worth worrying about.
Just to be clear, its one thing to receive a criminal charge, and quite another a conviction.
If all you had was just one ceid, for example, I think it would be exceptionally difficult to make a "manufacturing with intent to distribute" charge actually stick. I could see a threat, but I can't see a prosecutor actually trying to take that in front of a jury. In most cases the worst penalty you'd expect for mere possession of a ceid would be equivalent, say, to possession of a small amount of actual weed.
On the other hand, a pack of ceids + grow lights, pots and soil. . .arguably, that's "attempted manufacture". Throw in bags and a scale somewhere, and "attempted manufacture and intent to distribute" is plausible, though again, how likely an actual conviction would be depends on a whole bunch of factors.
Its against Federal law everywhere in the USA to possess even a single viable cannabis ceid, though again, in practice, the Feds have much bigger fish to fry than busting people over this. Customs is mostly looking for actual drugs and weapons, and they don't get terribly worked up over small numbers of ceids. . .assuming they even find them.
Note if customs does find your ceids, its their policy is to confiscate and destroy them. Almost by definition if they arrive at your door needing a signature, they haven't been detected.