I appreciate you taking the time to visit the Community. I'd be happy to provide some info on your question about an app for Square.
To view available applications for QuickBooks Online:
1. Click the Apps tab in the left navigation bar.
2. Search for Square.
Here you'll find several apps to choose from. I'm unable to recommend which would work best for your client's store but they'll be able to view all of the specifics for each application from this page. These can also be accessed by following this link: https://apps.intuit.com/app/apps/home.
Thanks for letting me know about the info you received on the Commerce sync.
Another app to consider is Sync with Square. This is the latest version of the Square QuickBooks Online integration. According to the overview, Square sales data is automatically imported into QuickBooks, eliminating the time spent manually entering this information.
I won't give you immediate help with my answer. However, there is an application named Business Payments that is going to be implemented with a feature of synchronization from Square to QuickBooks in their future updates. I believe that will be released in a few months. What I can suggest you do is to install their app and at the setup step click Square and leave your email to get to their list of users that going to be notified as soon as the feature is ready. Usually, the apps of this team are highly rated, so I believe that will become a nice working app.