Add a checkbox with a fee in woocomerce checkoute page.

functions.php

add_action( 'woocommerce_after_checkout_billing_form', 'add_box_option_to_checkout' );
function add_box_option_to_checkout( $checkout ) {
    echo '<div id="message_fields">';
    woocommerce_form_field( 'add_gift_box', array(
        'type'          => 'checkbox',
        'class'         => array('add_gift_box form-row-wide'),

        'label'         => __('Ilość pudełek ozdobnych - 25 PLN/szt'),
        'placeholder'   => __(''),
        ), $checkout->get_value( 'add_gift_box' ));


				woocommerce_form_field( 'add_gift_box', array(
						'type'          => 'checkbox',
						'class'         => array('add_gift_box form-row-wide'),

						'label'         => __('Ilość pudełek ozdobnych - 25 PLN/szt'),
						'placeholder'   => __(''),
						), $checkout->get_value( 'add_gift_box' ));
        echo '</div>';
}




// Update Cart
add_action( 'wp_footer', 'woocommerce_add_gift_box' );
function woocommerce_add_gift_box() {
    if (is_checkout()) {
    ?>
    <script type="text/javascript">
    jQuery( document ).ready(function( $ ) {
        $('#add_gift_box').click(function(){
            jQuery('body').trigger('update_checkout');
        });
    });
    </script>
    <?php
    }
}



add_action( 'woocommerce_cart_calculate_fees', 'woo_add_cart_fee' );
function woo_add_cart_fee( $cart ){
        if ( ! $_POST || ( is_admin() && ! is_ajax() ) ) {
        return;
    }

    if ( isset( $_POST['post_data'] ) ) {
        parse_str( $_POST['post_data'], $post_data );
    } else {
        $post_data = $_POST; // fallback for final checkout (non-ajax)
    }

    if (isset($post_data['add_gift_box'])) {
        $extracost = 25; // not sure why you used intval($_POST['state']) ?
        WC()->cart->add_fee( 'Ozdobne pudełka:', $extracost );
    }

}