Right, so the buyer pays the full price. Let's do it in simple terms. Your buyer buys 2 items at £10 each. Your postage is £4 for each item. So, ebay send the invoice to the buyer for £28. Ebay then deduct £8 from your transaction payment for the postage, so you get £20. You then refund the buyer £4 because you've sent the 2 items together. You are out of pocket surely.