How can I only sync orders that have been paid?

If you want to only sync order that have been fully paid, you can follow the steps below.

On the sync config, at the bottom you can setup Order filters to only sync orders that mach certain filter criteria. 

Add a new filter:

Select the "Financial status" criteria:

Select "paid" from the dropdown.  You can also select multiple values to combine them as an OR condition:

Make sure that the "Sync order updates" setting is enabled.  The app has to watch order updates in order to detect when the financial status has changed to paid.