@classmethod

    @transaction.atomic

    def manual_invoice(cls, user, due_delta, description_price_pairs):

        ''' Generates an invoice for arbitrary items, not held in a user's

        cart.

        Arguments:

            user (User): The user the invoice is being generated for.

            due_delta (datetime.timedelta): The length until the invoice is

                due.

            description_price_pairs ([(str, long or Decimal), ...]): A list of

                pairs. Each pair consists of the description for each line item

                and the price for that line item. The price will be cast to

                Decimal.

        Returns:

            an Invoice.

        '''

        line_items = []

        for description, price in description_price_pairs:

            line_item = commerce.LineItem(

                description=description,

                quantity=1,

                price=Decimal(price),

                product=None,

            )

            line_items.append(line_item)

        min_due_time = timezone.now() + due_delta

        return cls._generate(user, None, min_due_time, line_items)