I saw the case incentive too, but then I stumbled upon a preorder for it that appeared to be offered as a stand alone item which had me scratching my head.
This seems a common occurrence with these case incentives.
I suspect that technically, you shouldn't be able to pre-order or purchase the case-incentive on their own. In practice, what seems to happens is a store will place an order for X number of cases (to break them up and sell as booster boxes or bricks) so that way they can obtain a number of case-incentives and put them up for sale independent to the purchase of an entire case.
I obtained the case-incentive for the last set of Icon of the Realms miniature in this way, the Monster Menagerie II range which had Esmerelda's Wagon as the case incentive.