'Override to update carrier quotation if quantity changed'

@api.multi
def _cart_update(self, product_id=None, line_id=None, add_qty=0, set_qty=0, **kwargs):

    self._delivery_unset()
    self.write({'carrier_id': False})
    values = super(SaleOrder, self)._cart_update(product_id, line_id, add_qty, set_qty, **kwargs)
    if (add_qty or (set_qty is not None)):
        for sale_order in self:
            self._check_carrier_quotation()
    return values
