Either way you need to treat the snake cage you are seeing mites and any other cages that are near the infected one (The little buggers travel great distances)... You probably are getting reinfested because the problem has not been taken care of in the first place.... Its atleast a month long treatmant... You need to get them back down to paper and treat the paper... Do not put them back on the regular media until you are sure the problem is gone, throw out any unused media and dont buy any more until the problem is gone... Yes they will hitch are ride on anything... Try to go to someone else for any of your reptile needs, obviously the store has a problem and they will even hitch a ride on you(Snake mites will not harm anything but your snakes)

BTW wild snakes almost always have mites, they live together in the wild. The snake gets rid of the mites once it sheds and slithers away... The snake does not have this freedom in a cage and it becomes a vicious cycle if not treated... Its not the end of the world, you just need to take of the problem