They cannot advertise one thing and sell another, that's illegal. Call them on the false advertising and get the cockpit either credited off the price or included in your purchase.
Screen shot of the page showing its included is all you need to push the point.
I just had a two day battle with FedEx over this exact same thing with their site and the shipping system not being in sync.
You could make the purchase of the plane and dispute the charge I suppose but just the fact it's listed as included you should be able to hold them to it or its equivalent discount.