Seller demand more $$$$$ for postage costs almost a week after the sale & payment finalized?

[dylandnate]

I bid the minimum on an item last week, as always, taking into account how much the postage was.  I only bid the minimum, because if by some miracle I won, it was an amount I could afford.  I only ever bis what I can afford to immediately pay.  So when I bid on an item last week, I took into account how much the postage would be.  Based on this postage amount I made my bid to the seller (the absolute minimum/ first bid).  I would not have bid on this item if the postage were higher, as I did not have the money.  However, today, 5 days after winning and immediately paying for this item, I recieved a message from the seller stating the postage was double what was listed and that I was to pay the difference.  They did not ask if I was willing to do this, and as far as I can tell, they have gone ahead and posted.  As I'm on a VERY tight budget (single income family) I only had a total amount I could spare on this.  The extra amount being demanded, while seemingly a small amount, DOES actually make a HUGE difference to the running of our household. What I want to know, is the seller allowed to do this AND am I required to pay the difference, when, had I known the true cost, I would NEVER have bid on it????