Hello 'gold_butterfly',
You must have been shopping in the US. We don't (yet?) have the shopping cart feature here, but for those who continue to use ebay Canada, where sellers offer combined shipping the thing to do is buy the desired items then click the 'Request total from seller' link at the checkout. Then wait. So it has its disadvantages, - unless the discount happens automatically. Or a note to the seller will let him know what you are buying and he'll discount with the combined rate when you're done.
Either way, - happy spending! 🙂