To set up a one-time fee:
In the Custom Billing interface, select the domains you wish to apply a one-time fee to.
Click on "add one-time fees".
In the Add One-time Fees interface, enter the amount of the fee, a brief description of what the fee is for, and set the billing date. If you select a custom billing date, you must select a date that is later than the current date.
Click "ok". Your one-time fee has been applied to all selected domains.