I recently installed Google Customer Reviews on my Zen Cart plugins store. I used the Google Customer Reviews integration from Numinix, which I found quite satisfactory.
Google Customer Reviews basically takes over from Trusted Stores (which was sunsetted). You need to have a Google Merchant Center account to use it, and the threshold for having your reviews show up is pretty high (150 reviews), but I think it could be useful for some web shops.
One change I needed to make because my store is downloads-only: the file
./includes/templates/YOUR_TEMPLATE/jscript/jscript_google_customer_reviews.php
checks the delivery_country field in the orders object. I changed this to check the billing_country.
If you get prompted to take this Google survey after making a purchase from my store, I would appreciate you filling it out! Thank you.