Hi wiganyankeeron,
I'm not sure how things line up in the U.K., but it sounds like you are wanting to do your shipping like I do mine here in the U.S.A..
I select "Calculate" and then select the ways I want to allow shipping. I weigh the item and estimate the weight of the box and packing and enter that in the lbs and oz boxes. I will usually add a few ounces to be sure I am covered. At the bottom I will add whatever I need to cover my cost of boxes, packing and any labor. Since I handle all of that myself I only charge $2.00 S/H and then eBay will take care of the rest when a buyer looks at my item.
What I charge for shipping and the actual cost to the winner is usually within pennies of what the winner will pay and I am covered for my costs as well.
Hope this helps,
BroJames