Based on

CodePudding user response：

Since you don't want to add it to the total, you can add a custom table row to the woocommerce-checkout-review-order-table instead of a cart fee. So my answer is not based on the WooCommerce fee and is completely separate from it.

The custom table row will then show/hide the percentage, based on if the checkbox is checked.

Explanation via one-line comments, added to my answer.

So you get:

// Add checkbox field
function action_woocommerce_before_order_notes( $checkout ) {
    // Add field
    woocommerce_form_field( 'my_id', array(
        'type'          => 'checkbox',
        'class'         => array( 'form-row-wide' ),
        'label'         => __( '15% and some other text', 'woocommerce' ),
        'required'      => false,  
    ), $checkout->get_value( 'my_id' ));
}
add_action( 'woocommerce_before_order_notes', 'action_woocommerce_before_order_notes', 10, 1 );

// Save checkbox value
function action_woocommerce_checkout_create_order( $order, $data ) {
    // Set the correct value
    $checkbox_value = isset( $_POST['my_id'] ) ? 'yes' : 'no';
    
    // Update meta data
    $order->update_meta_data( '_my_checkbox_value', $checkbox_value );
}
add_action( 'woocommerce_checkout_create_order', 'action_woocommerce_checkout_create_order', 10, 2 );

// Add table row on the checkout page
function action_woocommerce_before_order_total() {
    // Initialize
    $percent = 15;
    
    // Get subtotal & shipping total
    $subtotal       = WC()->cart->subtotal;
    $shipping_total = WC()->cart->get_shipping_total();
    
    // Total
    $total = $subtotal   $shipping_total;
    
    // Result
    $result = ( $total / 100 ) * $percent;
    
    // The Output
    echo '<tr class="my-class">
        <th>' . __( 'My text', 'woocommerce' ) . '</th>
        <td data-title="My text">' . wc_price( $result ) . '</td>
    </tr>';
}
add_action( 'woocommerce_review_order_before_order_total', 'action_woocommerce_before_order_total', 10, 0 );
    
// Show/hide table row on the checkout page with jQuery
function action_wp_footer() {
    // Only on checkout
    if ( is_checkout() && ! is_wc_endpoint_url() ) :
    ?>
    <script type="text/javascript">
    jQuery( function($){
        // Selector
        var my_input = 'input[name=my_id]';
        var my_class = '.my-class';
        
        // Show or hide
        function show_or_hide() {
            if ( $( my_input ).is(':checked') ) {
                return $( my_class ).show();
            } else {
                return $( my_class ).hide();               
            }           
        }
        
        // Default
        $( document ).ajaxComplete(function() {
            show_or_hide();
        });
        
        // On change
        $( 'form.checkout' ).change(function() {
            show_or_hide();
        });
    });
    </script>
    <?php
    endif;
}
add_action( 'wp_footer', 'action_wp_footer', 10, 0 );

// If desired, add new table row to emails, order received (thank you page) & my account -> view order
function filter_woocommerce_get_order_item_totals( $total_rows, $order, $tax_display ) {    
    // Get checkbox value
    $checkbox_value = $order->get_meta( '_my_checkbox_value' );
    
    // NOT equal to yes, return
    if ( $checkbox_value != 'yes' ) return $total_rows;
    
    // Initialize
    $percent = 15;
    
    // Get subtotal & shipping total
    $subtotal       = $order->get_subtotal();
    $shipping_total = $order->get_shipping_total();
    
    // Total
    $total = $subtotal   $shipping_total;
    
    // Result
    $result = ( $total / 100 ) * $percent;
    
    // Save the value to be reordered
    $order_total = $total_rows['order_total'];
    
    // Remove item to be reordered
    unset( $total_rows['order_total'] );
    
    // Add new row
    $total_rows['my_text'] = array(
        'label' =>  __( 'My text:', 'woocommerce' ),
        'value' => wc_price( $result ),
    );
    
    // Reinsert removed in the right order
    $total_rows['order_total'] = $order_total;

    return $total_rows;
}
add_filter( 'woocommerce_get_order_item_totals', 'filter_woocommerce_get_order_item_totals', 10, 3 );