Ask your seller to use a PC /laptop/ desktop browser and to follow these instructions, after you have bought the item, to send you a revised invoice.
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 .
On the Seller Hub, click on the item in Orders >> Awaiting Payment, then top left you will see send Invoice, follow the above instructions.
They should then be able to amend the international postage unless this is a restricted item which cannot be sent overseas.
@wendyj14