I would think that if you're listing in a quantity of '1' per listing, you'd add s&h for 1 item. You can always send an invoice (to a buyer of multiples), or send a partial refund after the sale (remember that you don't get FVFs back on partial refunds) of multiples. Maybe another idea will come along. Good luck.
Since I have no idea the size/weight of your item, I will offer what I do for mine. I offer multiples once in awhile and pretty much they are always rather small and light weight. So, I set a flat rate for one with so much for each additional one.
