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If you are asking how to invoice a buyer for additional shipping charges, this is usually begun when the buyer requests an invoice (typically because you don't have an international shipping option and the buyer is international, or because the buyer purchased multiple items and is requesting a combined invoice). If the buyer has requested a different shipping service, then follow the instructions here:
https://www.ebay.com/help/selling/getting-paid/sending-invoices-buyers?id=4139
If the buyer has already paid, there is no option to re-invoice the sale. If the buyer is requesting a shipping method that is not present in your listing, your two options are to cancel the sale and relist with the requested shipping option, or to separately invoice the buyer through PayPal for the additional shipping.
