It is the fact known by sellers and not always understood by bidders,that eBay often does not give the automated save on postage with combined multiple wins mailing. So it is fine tip for buyers before paying for single item (even if You received the bill for won item!) to ask seller to wait for more of potential winning bids within soonest hours or days. After the winning of a few items make the request for total (automated payment with PayPal will just count all separate postages together) and most sellers (not all) will gladly combine the postage.
I also advice sellers to combine postage when it is possible,and trust me,this honesty will be valued by thankful buyers within many and many returns to Your items and many new purchases in future. Sellers might also be willing to mention along with the billing that buyer can wait for more items to end to save on mailing costs.