Convenience Charge option (check box) on a students transaction page

Having the option to add a convenience charge to select items to a students transaction. Example: I provide dancewear at my studio (that I do not include on my POS system, as its custom orders for students) & I would like to charge a convenience fee just for those specific charges. On a students page (on my account) - If i select "they owe you" I would love to have the convenience fee checkbox option.