This is the problem with Simple Delivery, sending two items using only one label, means you are at risk of buyer opening a Not received case for one item and getting a refund.
Long winded way to avoid this when buyer buys more than one item is to cancel sales and relist as one item for buyer to buy again, however delays in refunding for the cancelled sale and inconvenience to buyer may lose you two sales.
Another way is for buyers to put in basket before paying, but as buyers do not know or understand the complications this does not help at all, although you could put in the description, if buyers read and understand.
Really time ebay helped with a better system, but doubt it will happen.
