WooCommerce: Add a 4% Credit Card Processing Fee to the Checkout Page

woo-add-cart-fee.php

<?php
/**
 * WooCommerce Add Fee to the Checkout Page
 *
 * @package           WooCommerceAddFeeToCheckout
 * @author            Micah Wood
 * @copyright         Copyright 2023 by Micah Wood - All rights reserved.
 * @license           GPL2.0-or-later
 *
 * @wordpress-plugin
 * Plugin Name:       WooCommerce Add Fee to the Checkout Page
 * Plugin URI:        https://gist.github.com/wpscholar/52f928563d23b50287e9f1c4d0f8494b
 * Description:       Add a fee to the WooCommerce checkout.
 * Version:           1.0
 * Requires PHP:      7.4
 * Requires at least: 6.0
 * Author:            Micah Wood
 * Author URI:        https://wpscholar.com
 * Text Domain:       scratch-22
 * Domain Path:       /languages
 * License:           GPL V2 or later
 * License URI:       https://www.gnu.org/licenses/gpl-2.0.html
 */

add_action(
	'woocommerce_cart_calculate_fees',
	function () {
		global $woocommerce;
		$fee = ( $woocommerce->cart->cart_contents_total * 0.04 );
		$woocommerce->cart->add_fee( 'Processing Fee', $fee );
	}
);