I have always said on my listings that have a seperate postage charge that discounts are only given on items paid for on a single invoice so if they pay for items individually I assume they are not bothered by a discount and send the items out in different parcels just as if they were payments from different people. My store items have it clearly stated that there are no combined discounts on free post items aside from books all my store items have free post and have had since the start.