Is the address you have to send to a UK address?
If your items are listed with Ebay's Global Shipping Programme, you just have to send tracked to the UK address shown in 'order details'. So, no extra postage cost for you.
I haven't had an international sale for a while, but there may be some fees to pay.
Details here:
https://www.ebay.co.uk/help/selling/fees-credits-invoices/fees-private-sellers?id=4822&st=3&pos=1&qu...
International fee
If your registered address is in the UK, we charge an international fee of 3% if the delivery address for the item (entered by the buyer during checkout) is outside the UK.
This fee is calculated on the total amount of the sale and is automatically deducted from your sales proceeds. The total amount of the sale includes the item price, any handling charges, postage, and any applicable taxes.
If you don't want international buyers to purchase your items, you can learn how to exclude international delivery locations in our International selling policy.
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Søren Kierkegaard, Danish philosopher (1813 - 1855)