All in all, the cost is about the same if you order the spare with the truck or if you buy it aftermarket. (
http://www.bimmerzone.com/BMW_Perfor...-E70M-KIT.html)
But it's simplier to do it with the order, just make sure the dealer doesn't mess up. Make him show you all the options ordered in the system. You have to see SA300 spare tire