Woocommerce add markup to shipping rate

functions.php

<?php

function markup_shipping_rates($rates, $packages)
{
	$percentage = 5;

	$cartTotal = WC()->cart->get_totals()['cart_contents_total'];
	$additionalFee = ($percentage * $cartTotal) / 100;

	if (isset($rates['enviashipping:1'])) {
		$initialCost = $rates['enviashipping:1']->cost;

		$rates['enviashipping:1']->cost = $initialCost + $additionalFee;
	}

	return $rates;
}

add_filter('woocommerce_package_rates', 'markup_shipping_rates', 10, 2);