Hides the cost for free checkout add-ons

functions.php

/**
 * Hides the cost for free checkout add-ons
 *
 * @param string $html
 * @param object $fee
 * @return string
 */
function wc_hide_fee_cost( $html, $fee ) {
	$hide_for = array(
		'add-on-slug',
		// Add here as many add-ons as you need
	);

	if ( in_array( $fee, $hide_for ) ) {
		$html = '';
	}

	return $html;
}
add_filter( 'woocommerce_cart_totals_fee_html', 'wc_hide_fee_cost', 10, 2 );