Here's how to send an invoice with that revised postage cost, to include the international postage cost.
After your buyer has purchased the item, go right of the item sold to More Actions, and select Send payment Details, amend the International postage there to that required, but do not click on reset. Check it is correct, and send .
If you are on the Seller Hub, you click on the item in Orders >> Awaiting Payment, then top left you will see send Invoice, follow the above instructions.
You may need a PC /laptop and not the app or mobile browser to manage this. Chrome works well with eBay.
@christinoveren-0