Add Surcharge to Cart

cart-surcharge.php

/**
 * Add a 1% surcharge to your cart / checkout
 * change the $percentage to set the surcharge to a value to suit
 * Uses the WooCommerce fees API
 *
 * Add to theme functions.php
 */
add_action( 'woocommerce_cart_calculate_fees','woocommerce_custom_surcharge' );
function woocommerce_custom_surcharge( $cart ) {
    if ( is_admin() && ! defined( 'DOING_AJAX' ) ) {
        return;
    }

    $percentage = 0.01;
    $surcharge = ( $cart->cart_contents_total + $cart->shipping_total ) * $percentage;
    $cart->add_fee( 'Surcharge', $surcharge, true, '' );
}