As a buyer, you cannot change any detail on the seller's listing.
Have you purchased and paid for the item? Then, the seller cannot change those details now.
If there is an extra cost involved and you are happy to pay this, truly it was the seller's job to find out the best courier / price to sell their item beforehand, then you can pay this through a cheque , bank transfer, or a PayPal Friends and Family payment.
You are not obliged to pay this excess, as said , the seller should pay for any shortfall in a postage mistake they have made by miscalculation.
@dazugby