To be clear, the spores are perfectly legal. You can mail order spores that come in saline solution in a syringe without trouble. The businesses selling that are smart enough to not use media mail, which of course crosses a line.

The nuance here is you said “kit”, which suggests everything you need for growth and thus intent. Maybe you’re right on that bit. Buying everything separately would require surveillance to put all that together that you have a kit, in effect.