This feature syncs the total tax of the order as a line item as well as the duties as a line item in Xero invoice. To sync the duties as a line item is an optional.

This article explains how to setup QuickBooks Desktop integration for summary sync

This feature syncs the payment of the orders to separate Xero account when summary sync option is selected.

This order explains about order update sync from Shopify to QuickBooks.

This article explains about the cause of the above error.