Simple answer is, it doesn't. If you sell more than one item, Ebay add each item up at the maximum weight you selected. So if each item was under a kilo and you sell the same person 3 items, they get hit with 3 kilos worth of charges. If you offer free postage, you get hit with the extra charges.
Basically, Ebay have screwed this one up royally but as they get an extra couple of pence from each parcel sent via simple delivery... They couldn't care less.
