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