There is a problem with combining at the moment that ebay are supposed to be working on to fix. You could ask the buyer to pay in full and then issue a partial refund. That will complicate the tracking for the other items so you'd need to contact ebay to get the tracking 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 (if the buyer won them in auctions relist at the winning bid price) and a single postage cost for the buyer to buy and pay again.
