Have they paid? If they haven't then you can send them a revised invoice with a postage cost this way:
Go right of the item sold to More Actions, and select Send payment Details, amend the 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.
If they have paid, they need to send you the postage charge via a bank transfer, cheque or PayPal Friends and Family payment.
Not sure though how you then can mark the item as dispatched as it was purchased as a collection item and for this you need a code.
If they haven't received this code after payment, you could cancel the order citing Problem with Buyer's Account and relist the item for the buyer to select the postage option this time.
You will need a PC / laptop and not the app for the above.
@scouse-49
