We just set up two goals (sales targets) for each shop so you can track your conversion!
From the Piwik documentation:

Goal Conversion tracking is one of the most efficient ways to measure and improve your business objectives.

Read more about tracking goals and performance measurement here: http://piwik.org
Gather further information about marketing in line with our tour at shopmaker.com and feel free to contact us with any of your concerns regarding website marketing.