I have the same issue. and the link that you send only says "If you're using Google Analytics to track conversions for your eCommerce store, Squarespace sends the following information to Google Analytics for every received order"
But it doesn't seem to say how to get it to track conversions.
After doing some research it seem that an additional code needs to be added to the "Thank you for your purchase" page after the sale but I cant find anywhere what the code should be. The method that I have used to connect Google Analytics to my Squarespace shop has been through adding the google analytics account number to the external API Keys.
It is also only tracking everything on my page up until the checkout.
This is the information that I get from the Google analytics page when I try look up conversion rates.