QuickBooks Desktop by Parex

They are very clear and helpful. Just understand they are not Quickbooks, they are the app that connects your Shopify Store with Quickbooks Desktop.

They reply to all the emails the same day, the next day morning at the most.
The only not-so-good thing is that everything has to be via email. They don't accept calls/meetings. But if you are clear enough via email, they are very helpful.