Keep in mind that some sellers feel they make their money on shipping charges and they jealously guard those charges. In addition, they have to pay eBay 9% of the shipping charge as a commission, so they feel additional pressure to be slightly dishonest so they don't lose even more money.

Had the seller sent a revised invoice with combined shipping, though, eBay would have charged its commission on that revised figure rather than the postage quoted in the listing.

I'm not justifying what the seller did, because I don't operate that way.