Drupal 8 quick&dirty webform handler what creates Commerce 2 orders and sends user to checkout. Hard coded for event registration.

RegistrationProcessorWebformHandler.php

<?php

namespace Drupal\registrations_processor\Plugin\WebformHandler;

use Drupal;
use Drupal\commerce\PurchasableEntityInterface;
use Drupal\commerce_order\Adjustment;
use Drupal\commerce_order\Entity\OrderInterface;
use Drupal\commerce_price\Price;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Url;
use Drupal\views\Views;
use Drupal\webform\Plugin\WebformHandlerBase;
use Drupal\webform\Plugin\WebformHandlerInterface;
use Drupal\webform\WebformSubmissionInterface;
use Symfony\Component\HttpFoundation\RedirectResponse;

/**
 * Creates orders on webform submissions.
 *
 * @WebformHandler(
 *   id = "registrations_processor",
 *   label = @Translation("Registrations processor"),
 *   category = @Translation("Commerce"),
 *   description = @Translation("Creates orders on webform submissions."),
 *   cardinality = \Drupal\webform\Plugin\WebformHandlerInterface::CARDINALITY_UNLIMITED,
 *   results = \Drupal\webform\Plugin\WebformHandlerInterface::RESULTS_PROCESSED,
 *   submission = \Drupal\webform\Plugin\WebformHandlerInterface::SUBMISSION_REQUIRED,
 * )
 */
class RegistrationProcessorWebformHandler extends WebformHandlerBase {

  /**
   * Check if there are enough tickets.
   */
  public function alterForm(array &$form, FormStateInterface $form_state, WebformSubmissionInterface $webform_submission) {
    // TODO
  }

  /**
   * Populate commerce cart and go to checkout.
   */
  public function postSave(WebformSubmissionInterface $webform_submission, $update = TRUE) {
    if ($update) // Process only new submissions
      return;

    $data = $webform_submission->getData();

    // Volunteer
    if ($data['volunteer']) {
      return;
    }

    // Create order
    $order = Drupal::entityManager()
        ->getStorage('commerce_order')
        ->create(['type' => 'default', 'cart' => TRUE])
        ->setStore(Drupal::entityManager()->getStorage('commerce_store')->loadDefault());

    $order->setEmail($data['email']);

    $this->addProduct($order, $this->resolveTicket());

    if ($data['tshirt']) {
      $tshirt = Drupal::entityManager()->getStorage('commerce_product_variation')->loadBySku("T-shirt");
      $this->addProduct($order, $tshirt);
    }

    if ($data['bed']) {
      $bed = Drupal::entityManager()->getStorage('commerce_product_variation')->loadBySku("Bed");
      $this->addProduct($order, $bed);
    }

    if ($data['contrib']) {
      $order->addAdjustment(new Adjustment([
        'type' => 'custom',
        'label' => 'Contribution',
        'amount' => new Price($data['contrib'], 'CZK'),
      ]));
    }

    if ($data['student']) {
      $order->addAdjustment(new Adjustment([
        'type' => 'custom',
        'label' => 'Student',
        'amount' => new Price('-222', 'CZK'),
      ]));
    }

    $order->save();
    // Store the new cart order id in the anonymous user's session so that it
    // can be retrieved on the next page load.
    if (Drupal::currentUser()->isAnonymous()) {
      Drupal::service('commerce_cart.cart_session')->addCartId($order->id());
    }

    $this->redirectToCheckout($order->id());
  }

  protected function resolveTicket() {
    // List sold out tickets
    $view = Views::getView('tickets_sales');
    $view->execute();
    $sold_out = [];
    foreach ($view->result as $row) {
        $id = $row->_relationship_entities['commerce_product_variation']->id();
        $sold = (int)$row->order_item_id;
        $limit = (int)$row->commerce_product_variation_field_data_commerce_order_item__c;
        if ($sold >= $limit)
            $sold_out[$id] = TRUE;
    }

    $variants = Drupal::entityManager()->getStorage('commerce_product')->load(3)->getVariations();
    foreach ($variants as $v) {
        // If variant is not in sold_out
        if (!isset($sold_out[$v->id()]))
            return $v;
    }

    return FALSE;
  }

  protected function addProduct(OrderInterface $order, PurchasableEntityInterface $product_variation) {
    $item = Drupal::entityManager()
      ->getStorage('commerce_order_item')
      ->createFromPurchasableEntity($product_variation);
    $item->setQuantity(1);
    $item->save();
    $order->addItem($item);
  }

  protected function redirectToCheckout($order_id) {
    $url = Url::fromRoute('commerce_checkout.form', ['commerce_order' => $order_id]);

    $response = new RedirectResponse($url->toString());
    $request = \Drupal::request();
    // Save the session so things like messages get saved.
    $request->getSession()->save();
    $response->prepare($request);
    // Make sure to trigger kernel events.
    \Drupal::service('kernel')->terminate($request, $response);
    $response->send();
  }

}