You can add an extra fee to any invoice. This is very flexible and allows you to add a fee to an invoice without adding it to a specific course or class or applying it to every student purchasing the same thing. You can use this to create any kind of fee that you might use in courses and classes (like a Late Fee, Studio Fee or a Testing Fee) or any fee type other than a registration or materials fee.
On any invoice, you would just go to the "Add Items" drop down and select "Fee":
If you are using G|L accounts, select one in the G/L codes in the drop down. Type a name for the fee item, enter in a description for the fee and set an amount for the fee. When finished, click "Add a fee". Once done, the fee will display on the invoice.