It looks like a needle with a detachable tip, and will be most likely in a ziplock, and that will be inside a small box or air mail package.
I am not sure what you have to hide from, if someone asks you and you have to tell them its a mushroom spore, just say its a white button or something "edible". Spores are LEGAL in themselves. If you choose to germinate magic mushroom spores (and produce mushrooms via mycelium) then you can get in trouble. But in themselves no body can touch you.
There is a lot of preparation required that will definitely give you awawy if you are trying to be "stealthy"... the actual grow itself can be hidden, but yeah, the prep work cannot be done in a closest with a flashlight. You definitely need a small room like a bathroom and some space to work with.