To access financial information, you must have the 'Access billing information' global permission in your role.
This type of billing is not time-based. The amount specified in the box would be invoiced as a lump sum.
Project billing settings are used to produce billing reports.
When using the "Flat Fee" billing type—the billing portion of a metric/formula is added only after the project/activity is closed, unlike other billing types where the billing portion is updated as actual hours are added.