What app should I use to sync Square and Shopify sales into QBO. Square /QBO synchronizing is glitchy, so I prefer an app instead?

Check Synder

https://partnerstack.synder.com/quickbooks

At the moment, we can't recommend a specific app to sync your sales transactions from Square and Shopify with QuickBooks Online. However, you can consider SIAB's suggestion to check out Synder, Nerina. Currently, Amiko doesn't seem to support syncing transactions between Square and Shopify sales into QuickBooks Online.
 

To confirm, I recommend reaching out to Amiko's Support team for more details on whether they can integrate with QuickBooks Online (QBO).
 

For more information on finding apps that fit your sales transaction needs with QuickBooks Online (QBO), check out this article: Install and use apps in QuickBooks Online Accountant. While the article is intended for QBOA users, the steps for finding and installing apps also apply to QBO users.

Please don't hesitate to comment if you need more help.