Conversion Tracking Across Multiple Domains

I ran a search and did not find an answer to exactly what my issue is. Be easy…I’m not a techie.

We have several e-commerce websites. All of the websites use the same, single cart. So, it looks like this:> ---------------------^
etc… ----------------------------------^

How do you track this in Piwik? All of the websites and the cart are on the same server. I am not sure what additional information is needed to answer this question. We have installed the standard Piwik tracking code on the websites and the visitor tracking is working. This is the code we are using on the conversion page of our cart.

Can someone give me the correct code to use on the conversion page and let me know what (if anything) is needed for the non-cart tracking code?

Please elaborate if possible to help me understand all of the issues. Your help is incredibly appreciated.