Ebay collects fees on both postage and auction listing plus a percentage of the final selling price. Some Ebay auctions are listing free so you get a break, and Ebay collects a percentage of the other two charges (shipping and handling) and (final price paid). You have to be sure that you advise the potential buyer that you will give a discount if they meet certain requirement, and be sure they understand that you will have to adjust the invoice to reflect any discounts -- multiple purchases, shipping, you are a club member -- whatever you list as a bonus. It only works if the buyer doesn't pay for the items before you send an invoice. If that happens, the established postage fees that are listed in the calculator apply without any way to change other than to issue a refund if an overcharge. In the listing be sure to emphasize, if you are wanting any discount, you must wait for seller to send an invoice. Hope this doesn't confuse you more, but trust me it works every time. It's actually a selling tool allowing the seller to offer a discount on shipping for multiple auctions won (i.e. First chip ships for $3.00 all other won during this auction period, add $1) When the buyer is finished with the auctions, he will request through EBAY that you send a corrected invoice. Easy as pie!