How do I send invoice for postage only as item was pick up only but buyer now wants it posted and has already paid for item.
If you are happy to post, message the buyer and ask them to make a PayPal payment direct to your PayPal email of postage cost.
If they paid you by paypal go to the payment they made to you and get their PP address then send them a request for money.
Put in the request what it is for.
If they paid by bank dep then ask them for their PP details to send the request.