Hello,
The Sync device feature is a synchronization feature, not a backup feature.
The backup feature is available at "Tools > Back Up Data..." it will create a backup folder that can be restored by going to "Tools > Restore Data...".
This might not work, but you can try connecting your Express Invoice to Dropbox, when prompted, select to "Sync from the cloud", do not set your computer as a main device at it will replace the Dropbox data with the currently blank Express Invoice data.