I have just sold two items to the same buyer. Both items were separate in my Sold list and I wasn't aware that the same buyer had bought them both until I went to the first item and clicked on Send Invoice.

The Invoice then came up with both items and doubled the postage.  I just changed the postage and sent the Invoice.

They are now both combined in my Sold list. Buyer will pay one amount.

This is how I have always found the process to be.

Will have to see whether Paypal splits it.