import datetime

    @transaction.atomic

    def cancellation_fee(self, percentage):

        ''' Generates an invoice with a cancellation fee, and applies

        credit to the invoice.

        percentage (Decimal): The percentage of the credit note to turn into

        a cancellation fee. Must be 0 <= percentage <= 100.

        '''

        from invoice import InvoiceController  # Circular imports bleh.

        assert(percentage >= 0 and percentage <= 100)

        cancellation_fee = self.credit_note.value * percentage / 100

        due = datetime.timedelta(days=1)

        item = [("Cancellation fee", cancellation_fee)]

        invoice = InvoiceController.manual_invoice(

            self.credit_note.invoice.user, due, item

        )

        if not invoice.is_paid:

            self.apply_to_invoice(invoice)

        return InvoiceController(invoice)

    def test_cancellation_fee_is_applied(self):

        invoice1 = self._manual_invoice(1)

        invoice1.pay("Pay", invoice1.invoice.value)

        invoice1.refund()

        percentage = 15

        cn = self._credit_note_for_invoice(invoice1.invoice)

        canc = cn.cancellation_fee(15)

        # Cancellation fee exceeds the amount for the invoice.

        self.assertTrue(canc.invoice.is_paid)

        # Cancellation fee is equal to 15% of credit note's value

        self.assertEqual(

            canc.invoice.value,

            cn.credit_note.value * percentage / 100

        )

    def test_cancellation_fee_is_applied_when_another_invoice_is_unpaid(self):

        extra_invoice = self._manual_invoice(23)

        self.test_cancellation_fee_is_applied()