Guessing which item it is, I feel this was a genuine mistake on the sellers part, infact I even doubt they could post it for what they have charged. Being reasonably heavy and requiring extra packaging to avoid damage in transit.
But it is up to the seller to get the listing right before submitting it.
Beside the point any way as you bid expecting free postage, you can try to push the issue but can`t force them to send the item, I suggest you either negotiate a resolution or maybe request a mutual cancellation.
