If they've been bought by an overseas buyer through ebay's Global Shipping Programme you will not be able to combine. You have to send each item separately.
Otherwise click on More Actions > Send Payment Details, far right of one item on your sold page. 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. Don't tick "reset" afterwards, that will make it go back to original price, just submit.
