1. Assign the code IF to the field/column EXP/FEE/INV_ADJ_TYPE. This indicates an invoice-level adjustment on fees and, in this case, a flat fee.
2. Set LINE_ITEM_NUMBER_OF_UNITS to 1.
3. Set LINE_ITEM_UNIT_COST to the flat fee amount. This auto-calculates the amount as LINE_ITEM_UNIT_COST multiplied by LINE_ITEM_NUMBER_OF_UNITS.
4. Add a LINE_ITEM_DESCRIPTION to make sure you are aligned with client billing guidelines/expectations.
The following link is for LEDES.Org where you can see the requirements for file standards and examples: https://ledes.org/ledes-98bi-format/