Hey Dan,
It said the same thing to me. However, you still have buttons that allow you to go through with the purchase. It will also tell you that you need to remove it from your cart to proceed with the purchase, but you can just... proceed with the purchase anyway.
It should then give you an activation code. Plug that onto Steam, and you're good to go
Did that a week or so ago, and have been marvelling at the engine management in all its 15fps glory, hehehe!