add_action( 'woocommerce_after_checkout_billing_form', 'add_box_option_to_checkout' );
function add_box_option_to_checkout( $checkout ) {
  if ( WC()->cart->total > 0 ) {
	echo '<div id="message_fields">';
	woocommerce_form_field( 'add_gift_box', array(
		'type'          => 'checkbox',
		'class'         => array('add_gift_box form-row-wide'),
		'placeholder'   => '',
		'default' => 1
	), $checkout->get_value( 'add_gift_box' ));
	echo '</div>';}
}


/**
 * Add Javascript
 *
 * @version 1.0.0
 * @since   1.0.0
 */
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 fee to cart
 *
 * @link    https://docs.woocommerce.com/document/add-a-surcharge-to-cart-and-checkout-uses-fees-api/
 * @version 1.0.0
 * @since   1.0.0
 */
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 = $_POST;
	}
	if (isset($post_data['add_gift_box'])) {
		$extracost = 1;
		WC()->cart->add_fee( esc_html__( 'Gift Box:', '@@pkg.textdomain' ), $extracost );
	}
}