Depending on the circumstances you can choose to pay the extra postage money if you want to, but you are not obligated to. You are only obligated to fulfil the requirements of the listing at the time the transaction took place, which in this instance did not include the extra postage costs.
If you choose not to pay the extra costs, you should request a refund in the Resolution Centre. If the seller leaves you negative feedback at any stage, you can request for it to be removed.
cheers,
Adam
