If a buyer put the items in the basket first, it seems to then combine postage for them to pay at eBay checkout.
Some sellers are leaving a message on their listing, for example..... ' If you wish to buy multiple items, please let me know before buying, I will relist them as one listing so only one postage cost is charged, combining postage is not currently possible. Or, place items in your basket first, and eBay will combine a postage cost for you to pay. '
@spackygearflint