Add discount checkbox field to WooCommerce checkout, save meta data, display in admin and in email

custom-checkbox.php

<?php

// Add a custom checkbox fields after billing fields
add_action( 'woocommerce_before_order_notes', 'add_custom_checkout_checkbox', 20 );
function add_custom_checkout_checkbox(){

    // Add a custom checkbox field
    if (ICL_LANGUAGE_CODE == "hu") {
        woocommerce_form_field( 'discount_for_members', array(
            'type'  => 'checkbox',
            'label' => __(' Tagja vagyok az alábbi szervezetek egyikének: MKSSZ, IOKK, IGKS'),
            'class' => array( 'form-row-wide' ),
        ), '' );
    } else {
        woocommerce_form_field( 'discount_for_members', array(
            'type'  => 'checkbox',
            'label' => __('I am a member of one of the individual organizations: MKSSZ, IOKK, IGKS'),
            'class' => array( 'form-row-wide' ),
        ), '' );
    }
}

// Remove "(optional)" label
add_filter( 'woocommerce_form_field' , 'remove_order_comments_optional_fields_label', 10, 4 );
function remove_order_comments_optional_fields_label( $field, $key, $args, $value ) {
    // Only on checkout page for Order notes field
    if( 'discount_for_members' === $key && is_checkout() ) {
        $optional = '&nbsp;<span class="optional">(' . esc_html__( 'optional', 'woocommerce' ) . ')</span>';
        $field = str_replace( $optional, '', $field );
    }
    return $field;
}

// jQuery - Ajax script
add_action( 'wp_footer', 'checkout_fee_script' );
function checkout_fee_script() {
    // Only on Checkout
    if( is_checkout() && ! is_wc_endpoint_url() ) :

    if( WC()->session->__isset('enable_fee') )
        WC()->session->__unset('enable_fee')
    ?>
    <script type="text/javascript">
    jQuery( function($){
        if (typeof wc_checkout_params === 'undefined') 
            return false;

        $('form.checkout').on('change', 'input[name=discount_for_members]', function(e){
            var fee = $(this).prop('checked') === true ? '1' : '';

            $.ajax({
                type: 'POST',
                url: wc_checkout_params.ajax_url,
                data: {
                    'action': 'enable_fee',
                    'enable_fee': fee,
                },
                success: function (result) {
                    $('body').trigger('update_checkout');
                },
            });
        });
    });
    </script>
    <?php
    endif;
}

// Get Ajax request and saving to WC session
add_action( 'wp_ajax_enable_fee', 'get_enable_fee' );
add_action( 'wp_ajax_nopriv_enable_fee', 'get_enable_fee' );
function get_enable_fee() {
    if ( isset($_POST['enable_fee']) ) {
        WC()->session->set('enable_fee', ($_POST['enable_fee'] ? true : false) );
    }
    die();
}

// Add a custom dynamic 20% fee
add_action( 'woocommerce_cart_calculate_fees', 'custom_percetage_fee', 20, 1 );
function custom_percetage_fee( $cart ) {
    // Only on checkout
    if ( ( is_admin() && ! defined( 'DOING_AJAX' ) ) || ! is_checkout() )
        return;

    $percent = -20;

    if (ICL_LANGUAGE_CODE == "hu") {
        if( WC()->session->get('enable_fee') )
            $cart->add_fee( __( 'Kedvezmény tagoknak', 'woocommerce')." ($percent%)", ($cart->get_subtotal() * $percent / 100) );
    } else {
        if( WC()->session->get('enable_fee') )
            $cart->add_fee( __( 'Discount for members', 'woocommerce')." ($percent%)", ($cart->get_subtotal() * $percent / 100) );
    }
}

/**
 * Save & Display Custom Field @ WooCommerce Order
 */
 
// Save meta data
function save_new_checkout_field_discount_for_memberss( $order_id ) { 
    if ( $_POST['discount_for_members'] ) update_post_meta( $order_id, '_discount_for_members_checked', esc_attr( $_POST['discount_for_members'] ) );
}
add_action( 'woocommerce_checkout_update_order_meta', 'save_new_checkout_field_discount_for_memberss' );
   
// Display in admin order
function display_discount_for_members( $order ) {
    if ( get_post_meta( $order->get_id(), '_discount_for_members_checked', true ) == 1 ) {
        echo '<p><strong>Tagság:</strong> Tag vagyok</p>';
    } else {
        echo '<p><strong>Tagság:</strong> Nem vagyok tag </p>';
    }
}
add_action( 'woocommerce_admin_order_data_after_shipping_address', 'display_discount_for_members' );

// Display in emails
add_action( 'woocommerce_email_after_order_table', 'display_discount_for_members', 20, 4 );