If a buyer has an unpaid invoice with you, you can update the shipping costs on the order/invoice.
To update the shipping costs, locate the order in the Members Area > Selling > Sold Orders section, and click on the "Edit Invoice" button to the right of the order in question.
You can then update the shipping costs by editing the amount displayed in the "Postage Method" section:
Please note that it is against our terms to request additional shipping fees from a Buyer after an order has been made. The shipping costs should only be updated if you are providing the buyer with an additional discount, or you and the buyer have agreed on a different shipping method which may incur additional costs.
Additionally, please note that if a buyer has not yet made a purchase with you (as in they still have items in their shopping cart) you can not update the shipping costs on their order - as no order exists yet. You can however update the individual shipping costs on the items in question, by editing the listings themselves.