Ok, You have to put in a dollar amount for extra items. If someone wanted to purchase two or more of the items. You put in the shipping cost for the first one(item), and then put in a dollar amount for each item above the first. Example $5.00 for first item, then each additional item $2.50 for shipping. It's only for that item and not other auctions. Enter the quantity, then go to shipping the Additional cost area will show up automatially when you enter more than "1" in the quanity field. Hope this is clear for you.
Also you can edit your post. Look at your post, in the bottom right hand corner next to the little quote thingy click on the little icon and you can edit/delete your post.