I also offer combined postage, but in reality, about 60% of my buyers only purchase one item.

For the other 40%, it's better to have them pay and refund a small amount of postage for the combined items, than not have payment at all.

Sometimes you'll find that no refund is necessary because by the time you have weighed and packed those bulky plastic items, there's nothing to refund. I can see that many of your items are different shapes, so they cannot necessarily be stacked one inside the other for postage.