Please see the following information regarding setting up an export template to allow for grouping and subtotaling:
The fields must appear in both the Detail and Group Footer areas. It is necessary to rename the Footer fields to allow the export to run.
The detail fields must remain unchanged.
Value fields such as Gross should show only in the Detail if a sum is required - otherwise the report will group by that too.
If you require column headers in your export document then add Fixed Value fields to the File Header and rename them as required.