You cannot add the item to the invoice without a legitimate ebay transaction.
You will need to relist the item. You can list it as a buy it now price that you and the buyer agree on. Once they purchase the item, you can re-submit the invoice and it will include the new item. If you need to make adjustments to shipping, DO NOT click on recalculate. Instead, click on preview to make sure everything is accurate then click Send.
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Non payers should get unpaid item strikes and be added to block bidders lists. Period.