Put each of those 13 item into your Shopping Cart. (You will need to be at a desktop to do it.)
What amount do you see for postage? That is the amount you should expect to pay. You can Request Invoice From Seller but he's not obligated to lower the cost you see in the Shopping Cart. If you have expectations that it should be lower, you must confirm that to be true with the seller prior to purchase.
Some sellers don't offer combined shipping. This could be for many reasons: the items might be too bulky, or they might ship from different locations, or the seller might count on postage handling fees to make their profit.
Did the seller say he couldn't combine postage? Or that he wouldn't? It's not the same thing. What do the listings themselves say about combined shipping? Is it offered as a lure in the text of the page anywhere?
Personally, as a seller, I love to combine shipping and I don't charge anything for handling but everyone here is independent and has their own business model.
