You cannot change the delivery option after the item has been sold, and you cannot adjust the postage if the buyer has already paid you.
Have they paid? Then, they need to send you payment for that postage via a cheque, bank transfer, or PayPal Friends and Family payment if they have a PayPal account.
Or, you can send them a PayPal invoice for the amount, they will not need a PP account for this, and can pay via various card options.
Select the method which suits you both best.