As a mainly seller, it is VERY frustrating. I always like to combine s/h. But I do realize many buyers just pay for separate items, for reasons stated above. I then have to refund the extra s/h. That's basically the only way I can figure out making things right.
I'm usually not a negative person, but I think ebay does this on purpose. Once they made the decision to take a small percentage of s/h fees, they make it hard to combine s/h. As they will make a bit more from individual purchases.
It actually has gotten so frustrating to me, now I do Free S/H on all my listings. So now, a buyer can buy as many cards, and I don't have to worry about it any more.
Not fun, Scott