If you have an agreement with an employer that they will pay a flat fee regardless of how many employees are enrolled, you can follow these steps to set up their plan.
Click on the tab for 'Employer 'Plans'
Click '+ New Plan'
Name the plan whatever you'd like and make sure the field 'Fixed fee' is selected
As with all other plans you can select if you'd like the billing period to be monthly, annual, semi-annual, or quarterly
Set the amount that should be billed each billing period
You can then continue with these directions for setting up a new employer or adding this plan to an existing employer.