Depends on what you are selling.
I offer a discount on items I sell if more than one item is purchased.
Unfortunately you cannot please everyone and some will mark you down on poatage costs on the ratings like I have been sometimes.
People don't seem to realise the postage costs these days and particularly on smaller items they think postage is expensive for them but we don't set the postage costs Royal Mail does and they are the greedy ones.
I always keep postage costs as low as I can but like every other seller you need to cover your postage costs and packing materials to send out items.
Unlike years ago when postage was cheap these days it's not and even sending a large birthday card isn't cheap!