An e-commerce module enables tracking of business. Thanks to this you know which traffic source brings the best cusommers, which pages has the biggest impact to conversions or which products are "top sellers".
When a custommer changes his mind and cancel his purchase, data are just sent to Google Analytis servers and it is impossible to remove it completly. Analyst see some sales in his GA reports, which were not accomplished.
The solution is simple - it is necessary to use a "negative transaction". It is a transaction with total amount, tax and shipping are negative values, count of all items is negative also. I. e. when a custommer purchase 1 T-shirt for 10 USD, you have to track a negative transaction with -1 T-shirt with amount of -10 USD. Google Analytics deducts this amounts and consequest sales and sold item count will correspond with reality. You must however keep in mind that the number of transactions is not number of deals, but it is higher.
If you use a GA script, you can use identical notation for an any number of created trackers:
Example for using plain GA tracking code is in Google documentation.