And it may be and I just may be over looking it. But I will use screenshots from your demo site since it has more sample content then my test server has.
In the case of JS Fashion in the screenshot. They notify us that they do not need the Nikon D3200 they placed the order for and would like to cancel that but keep the rest. But I do not see a way to cancel that specific order line.
and another example:
The decided after they placed the order of the Lava V2 3GB Ram (Icy White, 16 GB). That they only needed 1 and not 2. I do not see a way to edit line quantities either.