what are my rights when sent an invoice with postage costs almost double those quoted on items bid on?

[quentaris]

I bid on three items from a seller, checking first to see if they combined postage. After receiving an affirmative response, i bid and won three items. Upon requesting an invoice the seller has almost doubled the postage charge quoted for the items (maximum postage for three items according to their listings was gbp 24.50 and i have been quoted gbp 42.30. 

This is grossly excessive for the items and i want to know where i stand. None of the items are heavy (a game, a magazine and a jigsaw - the heaviest of the three had a postage charge of gbp 8.50 in the listing)

I am now being asked to pay gbp 14 per item.