On My eBay click on Send Payment Details, far right of one item on your sold page (you may have to click on "More Actions" first). The other items will be on there. Make sure they're ticked to show you're combining, then change the postage to what you've decided to charge and submit (don't click on reset if it's there).
Or on seller hub go to the Orders tab, tick the box beside the orders that need to be combined and select Send Invoice for the orders selected.
