I strongly recommend to play with the demo component. With the sample you will maybe understand better the logic.
What do you mean when you say you were not authorized ? For the demo ?
Let me explain a bit.
displayXxxx(), is the rendering View function.
'Xxxx' is the alias name of your layout. it can be displayDefault() for instance.
First, choose the correct layout page for this calculation, then edit it for achieving the calculation.
For a novice, it is the most easy. Then if you are a purist, you can place this new function in the model or somewhere else. At the moment, just try to achieve it in the view file, which is the most easy.
Todo : Take your items lists ($this->items) into a loop for summing every item value. If you really don't understand, I can help you more.
Otherwise, you can hire a developer here :
www.j-cook.pro/index.php/support/23-need-a-developper