I have just come across this with my existing listings, as I have just found out that they change to SD if I make any changes on existing listings.  My issue is with this first SD sale is that I have sold 5 of one item (different designs) and SD have only charged one postage cost as if I was selling one item, so I hope that my buyer will not be charged if it costs any more to ship 5 items.  From what I understand as long as it is in a box of a certain size and under 2kg it can be sent, but I don't have a box that size to fit 5 items... I'll have to see what happens and contact ebay accordingly if I have to refund my buyer extra postage costs.