Once you click Buy or Bid on eBay, you have agreed to the seller's terms as listed in the auction, assuming their terms are in harmony with eBay's and PayPal's overall user agreements and terms.
So, if there was nothing in the auction details about the seller combining shipping charges, they are under no obligation to do so under either the eBay or PayPal rules.
The correct thing to do would have been to click the "ask seller a question" button before clicking the Buy It Now button and asked the seller if they will combine shipping for a discount and if so provide the exact costs, just as they did for each item purchased stand-alone.