There is a problem with combining at the moment.  You could ask the buyer to pay in full and then issue a partial refund.  That will complicate the tracking so you'd need to contact ebay to get it correct on all the items.  You can use callback on here:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid 

An alternative would be to cancel the transactions (a refund would be issued) and relist with items on one listing and a single postage cost for the buyer to buy and pay again.