How do I add a fee to the invoice?

In order to add a fee to WooCommerce and your invoice, simply add the following action to your themes  functions.php

function add_woocommerce_fee() {
    global $woocommerce;

    if ( is_admin() && ! defined( 'DOING_AJAX' ) )

    $amount = 5;
    $woocommerce->cart->add_fee( 'FEE_NAME', $amount, true, 'standard' );
add_action( 'woocommerce_cart_calculate_fees','add_woocommerce_fee' );

Please test the functionality on a development environment of your website. If it ceases to work, please revert back to the original state of your website and follow the tutorial again.