class Order extends React.Component {
  ...
  render() {
    return <div>
      <strong>Total: $ {this.getTotal()}</strong>
      <table id='orderlines' style={tableStyles}>
        <tbody>
          {this.getLines()}
        </tbody>
      </table>
    </div>
  }

  getLines = () => {
    return this.state.orderLines.map((item, _) =>
      <OrderLine key={item.id} {...item} />)
  }
}

getTotal = () => (this.props.product.price * this.state.quantity).toFixed(2);