You need to use a PC / laptop and use Chrome or Firefox for this they work best, then this is how to manage a combined invoice.
Go right of the item sold to More Actions, and select Send payment Details all items will appear on the one invoice, 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.
