So an item listed at $15 (which includes $7 post) is fine for 1 item but start muliplying that and it gets nasty.

Yes, it does. Would put the buyer off buying more than one, unless the seller has written in the description that they offer reduced price for combined items, but only  if the buyer requests a new combined invoice BEFORE they make payment.

On free post items the seller is unable to offer any form of discount on the invoice.