WooCommerce send order complete email automatically
You can use woocommerce_order_status_completed hook and WC_Email_Customer_Invoice
add_action( 'woocommerce_order_status_completed', 'mysite_woocommerce_order_status_completed', 10, 1);
function mysite_woocommerce_order_status_completed( $order_id ) {
// Send invoice email to customer
//use order ID as trigger value
WC()->mailer()->emails['WC_Email_Customer_Invoice']->trigger($order_id);
}