just go into your sold folder and on the left hand side it should see a button that says send invoice, if not click on the more options drop down menu and click send invoice or resend invoice. open it up, add the postage but do not hit recalculate. if you want to check it scroll to the bottom and near the send button there is a link to preview.
if the buyer has alreay paid for the item then you will have to send a paypal request/invoice for money direct from paypal, log into paypal and click on the send request money tab.
