If I'm right, when you order for a saber with Obsidian sound, the cable is provided. The $3 cable on the site might be a remplacement cable in case you blew up the one you get with the saber.
I say that regarding that if you buy the board itself the cable is provided :
http://www.ultrasabers.com/product-p/z-sound-obsidianusb.htm"Come with mini USB cord"
But I might be wrong, if anyone that have his Obsi-board saber can confirm it (or US staff), this will avoid some people to purchase a cable they already would have.