Your buyer will need to pay for the items together as one payment and eBay should then combine postage and issue you one label, if this doesn't happen you can cancel with buyers permission and relist both items on a new listing for the buyer to repurchase.
If you send as one parcel with both items using the original label you wont have proof of delivery for the second item, the buyer would be refunded the postage in around 30 days and your payment iss likely to be held for 14 days.
