yes. the are legal in every country of the world, except for Denmark (i think).
BUT that isnt to say that some jurisdiction may class them as "Drug Paraphenalia". This catch all phrase could include almost everything, tho, so dont stress it. If something is classed as paraphenalia is usual isnt illegal in its self, but can be used to prove "probable cause" for a search etc.
the bottom line is it is legal to purchase them, online or in person, in every state of the union. it is legal to have one in your possesion, altho it may draw unwanted attention.
AFAIK all test kit suppliers send out their products in a plain, unmarked package that gives no indication of it's contents.