I recommend waiting until they get it back in stock, then contact customer service to ask why your order was canceled due to 'lack of availibility'

I read a similar thing happened when they had Iron Man 2 for preorder for $15, they canceled orders due to 'lack of availibility' and raised the price to $25. It was later clarified as a pricing error, but some who contacted customer service got a $10 credit to buy it at the originally advertised price.

While I doubt Walmart will honor the original price, they might still acknowledge their mistake.