How do you know the total number of each product that you’ve sold each month on Shopify?

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Yes, the best way to do it is with a Shopify app. I actually don’t know why Shopify don’t just enable to export a report, it is such an essential feature.

There’s an app to integrate Quickbooks if you use that for your accounts.

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If you want to get the reports from Shopify then you’ll probably need to upgrade your plan.
I actually think that an app is the preferable solution. There are apps for Quickbooks and Xero integration. I personally use Quickbooks, their app syncs orders to my accounting system and pulls the data from my accounting system.