In general invoices for a flat fee matter will come from a blank invoice with a service for the flat fee. However, if you are looking to also add soft and hard cost expenses to the invoice as well you will need to create the invoice from unbilled activities first then add the service line items for the flat fee:
- Create all Soft and Hard costs for the Matter. (Make sure to select Billable)
- Generate an invoice from Unbilled Activities.
- Go to the invoice and add a new line item for the Flat Fee as a Service