One of the most valuable things in Google Analytics is the ability to tie an actual transaction value to campaigns and other activities. Unfortunately, Google Analytics doesn't take into account refunds, returns or cancellations. If there are enough of these, you may find that your ecommerce reports don't line up with reality.
The good news is there is a clean way of editing transactions after they have been reported in Google Analytics. It requires some custom coding, but it will bring your reports closer to reality.