When I see the 'seller doesn't offer combined shipping' nonsense I use the 'contact the seller' option to request a combined invoice.
Except that the seller can't see "Buy It Now" items added to the cart. The seller only knows of the winning auction items. So I can't put 10 items in my cart and then ask for a combined invoice. (edit: I can attempt
to request a total for the combined invoice but get the erroneous error quoted above).
Sometimes I get the message "You cannot request a combined invoice because the seller has already offered a discount on one or more of these items." So because one of the items was on sale and I got a ten percent discount on a $2 item, I saved twenty cents on my order but because of that I have to pay $10 total postage since I can't combine shipping? Nah.
Agreed, just use the Contact Seller option. If they can't provide the combined invoice, the seller sometimes says just pay the whole thing and they will reimburse. I can't remember a time when the seller didn't provide a refund of the excessive postage.
If the seller refuses to refund excess postage and causes shenanigans, it will cost them on their feedback. If the seller already has low feedback, then it is buyer beware.