add-a-surchages-to-cart-and-checkout-page.php

/**
 * Add a 1% surcharge to your cart / checkout.
 * change the $percentage to set the surcharge to a value to suit.
 */
function wb_woocommerce_custom_surcharge() {
	global $woocommerce;

	if ( is_admin() && ! defined( 'DOING_AJAX' ) ) {
		return;
	}

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