Automatic Discount on a Customer's First Order

$wpide_first_order = array(
    'percent' => 10,                     // Percent off the products subtotal.
    'label'   => 'First order discount', // Line label.
);

add_action( 'woocommerce_cart_calculate_fees', function ( $cart ) use ( $wpide_first_order ) {
    if ( ( is_admin() && ! wp_doing_ajax() ) || ! WC()->customer || ! WC()->session ) {
        return;
    }

    $email = is_user_logged_in() ? wp_get_current_user()->user_email : WC()->customer->get_billing_email();

    if ( ! is_email( $email ) ) {
        return;
    }

    $cache = (array) WC()->session->get( 'wpide_first_order', array() );
    $key   = md5( strtolower( $email ) );

    if ( ! isset( $cache[ $key ] ) ) {
        $previous = wc_get_orders( array(
            'billing_email' => $email,
            'status'        => wc_get_is_paid_statuses(),
            'limit'         => 1,
            'return'        => 'ids',
        ) );

        if ( ! $previous && is_user_logged_in() ) {
            $previous = wc_get_orders( array( 'customer_id' => get_current_user_id(), 'status' => wc_get_is_paid_statuses(), 'limit' => 1, 'return' => 'ids' ) );
        }

        $cache[ $key ] = empty( $previous );
        WC()->session->set( 'wpide_first_order', $cache );
    }

    if ( $cache[ $key ] ) {
        $cart->add_fee( $wpide_first_order['label'], -$cart->get_subtotal() * $wpide_first_order['percent'] / 100, true );
    }
} );