SMMware allows you to add fees to a job where the fees are not based on a number of hours, but rather a flat amount.
Example 1: If you charge $50.00 for packing materials.
Example 2: If you charge a $75 scheduling fee for each day of work on a job.
This is typically used where the flat fee is not the whole job, but just another line item. If the fee you have in mind does amount to the whole job, you probably want to just use an invoice of type "Flat Amount" (as opposed to an itemized invoice).