GACO: working grat

add_action( 'woocommerce_cart_calculate_fees', 'custom_fee_based_on_cart_total', 10, 1 );
function custom_fee_based_on_cart_total( $cart_object ) {

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

    // The percetage
    $percent = 15; // 15%
    // The cart total
    $cart_total = $cart_object->cart_contents_total;

    // The conditional Calculation
    $fee = $cart_total > 0.01 ? $cart_total * $percent / 100 : 0;

    if ( $fee != 0 )
        $cart_object->add_fee( __( "Gratuity", "woocommerce" ), $fee, false );
}