How do I programmatically "place" an order?
A little digging and I figured it out on my own:

Event handler for for this event: commerce_order.order.paid

and with this handler code: 

  public function paymentCompletedHandler($event) {
    $order = $event-&gt;getOrder();
    $order-&gt;getState()-&gt;applyTransitionById('place');
    return;
  }