You can add a goal to your analytics account for each sale that is processed through your Rocketspark website.
You can add a goal to capture the traffic that is passing through your checkout finish page URL (also called the checkout success page). Traffic will only go to this URL once they have completed the purchase process /shop/checkout/finish/
For example: www.mywebsiteaddress.com/shop/checkout/finish/
By using this URL in your Google Analytics goal you will capture the customers that have selected an item and have completed purchasing it through your website's payment methods.
You can also add specific conversion tracking to your checkout finish page which is useful when using paid digital advertising such as adwords.
See our guide on adding checkout success conversion tracking to your checkout finish page
See our guide on Google Analytics event tracking
See Google's guides for setting up goals in Google Analytics