Question

How to export Invoice data and LineItems into a single file?

Answer

As known, the FlexiCapture exports the repeatable instances into separated files.

However, it can be avoided by the option Make separate files for repeatable items turned off inside export options: 

This way, all data will be written to a single line.

If the data should be transformed to a multiline view the export script is the only way: https://help.abbyy.com/en-us/flexicapture/12/developer/script_export/