The problem with a poll at checkout is that, as I mentioned earlier, it depends on what bit is backordered.

As an extreme example,

I order an engine and some cyano but the cyano is not in stock. I would want the engine straight away but would cancel the cyano order.

I order the same items but the engine is not in stock. Now my choice would be different; I would want to cancel the whole order.

Communication is the key.