Combined invoices payments come up seperate

[xena200]

Hi does anyone have the same problem,sometimes when i send invoices payments are not combined even though items are in auction and not buy it now,buyer askes for the postage difference to be refunded but ebay and paypal don`t refund the extra charge on postage and paypal charge the extra 40cents because of seperate transaction,adds up to around 70cents extra each time,my items are automatically supposed to be combined,it happened twice today,once in auction when a buyer bought 2 lots even though i sent a combined shipping invoice,also in my store when a buyer bought 2 items,it happens often but can`t work out why thanks