diff --git a/modules/checkout/src/CheckoutFlowPluginCollection.php b/modules/checkout/src/CheckoutFlowPluginCollection.php
deleted file mode 100644
index c7b8ba984d..0000000000
--- a/modules/checkout/src/CheckoutFlowPluginCollection.php
+++ /dev/null
@@ -1,62 +0,0 @@
-<?php
-
-namespace Drupal\commerce_checkout;
-
-use Drupal\Component\Plugin\Exception\PluginException;
-use Drupal\Component\Plugin\PluginManagerInterface;
-use Drupal\Core\Plugin\DefaultSingleLazyPluginCollection;
-
-/**
- * A collection of checkout flow plugins.
- */
-class CheckoutFlowPluginCollection extends DefaultSingleLazyPluginCollection {
-
-  /**
-   * The checkout flow entity ID this plugin collection belongs to.
-   *
-   * @var string
-   */
-  protected $entityId;
-
-  /**
-   * Constructs a new CheckoutFlowPluginCollection object.
-   *
-   * @param \Drupal\Component\Plugin\PluginManagerInterface $manager
-   *   The manager to be used for instantiating plugins.
-   * @param string $instance_id
-   *   The ID of the plugin instance.
-   * @param array $configuration
-   *   An array of configuration.
-   * @param string $entity_id
-   *   The checkout flow entity ID this plugin collection belongs to.
-   */
-  public function __construct(PluginManagerInterface $manager, $instance_id, array $configuration, $entity_id) {
-    parent::__construct($manager, $instance_id, $configuration);
-
-    $this->entityId = $entity_id;
-  }
-
-  /**
-   * {@inheritdoc}
-   *
-   * @return \Drupal\commerce_checkout\Plugin\Commerce\CheckoutFlow\CheckoutFlowInterface
-   *   The checkout flow plugin.
-   */
-  public function &get($instance_id) {
-    return parent::get($instance_id);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function initializePlugin($instance_id) {
-    if (!$instance_id) {
-      throw new PluginException("The checkout flow '{$this->entityId}' did not specify a plugin.");
-    }
-
-    $configuration = ['_entity_id' => $this->entityId] + $this->configuration;
-    $plugin = $this->manager->createInstance($instance_id, $configuration);
-    $this->set($instance_id, $plugin);
-  }
-
-}
diff --git a/modules/checkout/src/Entity/CheckoutFlow.php b/modules/checkout/src/Entity/CheckoutFlow.php
index 030f0db866..785a061b8d 100644
--- a/modules/checkout/src/Entity/CheckoutFlow.php
+++ b/modules/checkout/src/Entity/CheckoutFlow.php
@@ -2,7 +2,7 @@
 
 namespace Drupal\commerce_checkout\Entity;
 
-use Drupal\commerce_checkout\CheckoutFlowPluginCollection;
+use Drupal\commerce\CommerceSinglePluginCollection;
 use Drupal\Core\Config\Entity\ConfigEntityBase;
 
 /**
@@ -83,7 +83,7 @@ class CheckoutFlow extends ConfigEntityBase implements CheckoutFlowInterface {
   /**
    * The plugin collection that holds the checkout flow plugin.
    *
-   * @var \Drupal\commerce_checkout\CheckoutFlowPluginCollection
+   * @var \Drupal\commerce\CommerceSinglePluginCollection
    */
   protected $pluginCollection;
 
@@ -138,13 +138,13 @@ public function set($property_name, $value) {
    *
    * Ensures the plugin collection is initialized before returning it.
    *
-   * @return \Drupal\commerce_checkout\CheckoutFlowPluginCollection
+   * @return \Drupal\commerce\CommerceSinglePluginCollection
    *   The plugin collection.
    */
   protected function getPluginCollection() {
     if (!$this->pluginCollection) {
       $plugin_manager = \Drupal::service('plugin.manager.commerce_checkout_flow');
-      $this->pluginCollection = new CheckoutFlowPluginCollection($plugin_manager, $this->plugin, $this->configuration, $this->id);
+      $this->pluginCollection = new CommerceSinglePluginCollection($plugin_manager, $this->plugin, $this->configuration, $this->id);
     }
     return $this->pluginCollection;
   }
diff --git a/modules/checkout/src/Form/CheckoutFlowForm.php b/modules/checkout/src/Form/CheckoutFlowForm.php
index a7864577f4..654be162b7 100644
--- a/modules/checkout/src/Form/CheckoutFlowForm.php
+++ b/modules/checkout/src/Form/CheckoutFlowForm.php
@@ -4,7 +4,6 @@
 
 use Drupal\commerce\Form\CommercePluginEntityFormBase;
 use Drupal\commerce_checkout\CheckoutFlowManager;
-use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
@@ -45,6 +44,12 @@ public function form(array $form, FormStateInterface $form_state) {
     $checkout_flow = $this->entity;
     $plugins = array_column($this->pluginManager->getDefinitions(), 'label', 'id');
     asort($plugins);
+    // Use the first available plugin as the default value.
+    if (!$checkout_flow->getPluginId()) {
+      $plugin_ids = array_keys($plugins);
+      $plugin = reset($plugin_ids);
+      $checkout_flow->setPluginId($plugin);
+    }
 
     $form['#tree'] = TRUE;
     $form['#attached']['library'][] = 'commerce_checkout/admin';
@@ -63,7 +68,7 @@ public function form(array $form, FormStateInterface $form_state) {
       ],
     ];
     $form['plugin'] = [
-      '#type' => 'select',
+      '#type' => 'radios',
       '#title' => $this->t('Plugin'),
       '#options' => $plugins,
       '#default_value' => $checkout_flow->getPluginId(),
@@ -80,18 +85,6 @@ public function form(array $form, FormStateInterface $form_state) {
     return $this->protectPluginIdElement($form);
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  protected function copyFormValuesToEntity(EntityInterface $entity, array $form, FormStateInterface $form_state) {
-    /** @var \Drupal\commerce_checkout\Entity\CheckoutFlowInterface $entity */
-    // The parent method tries to initialize the plugin collection before
-    // setting the plugin.
-    $entity->setPluginId($form_state->getValue('plugin'));
-
-    parent::copyFormValuesToEntity($entity, $form, $form_state);
-  }
-
   /**
    * {@inheritdoc}
    */
diff --git a/modules/payment/src/Entity/PaymentGateway.php b/modules/payment/src/Entity/PaymentGateway.php
index 072b2f772f..f93a1fbaae 100644
--- a/modules/payment/src/Entity/PaymentGateway.php
+++ b/modules/payment/src/Entity/PaymentGateway.php
@@ -2,9 +2,9 @@
 
 namespace Drupal\commerce_payment\Entity;
 
+use Drupal\commerce\CommerceSinglePluginCollection;
 use Drupal\commerce\ConditionGroup;
 use Drupal\commerce_order\Entity\OrderInterface;
-use Drupal\commerce_payment\PaymentGatewayPluginCollection;
 use Drupal\Core\Config\Entity\ConfigEntityBase;
 
 /**
@@ -113,7 +113,7 @@ class PaymentGateway extends ConfigEntityBase implements PaymentGatewayInterface
   /**
    * The plugin collection that holds the payment gateway plugin.
    *
-   * @var \Drupal\commerce_payment\PaymentGatewayPluginCollection
+   * @var \Drupal\commerce\CommerceSinglePluginCollection
    */
   protected $pluginCollection;
 
@@ -247,13 +247,13 @@ public function set($property_name, $value) {
    *
    * Ensures the plugin collection is initialized before returning it.
    *
-   * @return \Drupal\commerce_payment\PaymentGatewayPluginCollection
+   * @return \Drupal\commerce\CommerceSinglePluginCollection
    *   The plugin collection.
    */
   protected function getPluginCollection() {
     if (!$this->pluginCollection) {
       $plugin_manager = \Drupal::service('plugin.manager.commerce_payment_gateway');
-      $this->pluginCollection = new PaymentGatewayPluginCollection($plugin_manager, $this->plugin, $this->configuration, $this->id);
+      $this->pluginCollection = new CommerceSinglePluginCollection($plugin_manager, $this->plugin, $this->configuration, $this->id);
     }
     return $this->pluginCollection;
   }
diff --git a/modules/payment/src/PaymentGatewayPluginCollection.php b/modules/payment/src/PaymentGatewayPluginCollection.php
deleted file mode 100644
index 6aff7d1f28..0000000000
--- a/modules/payment/src/PaymentGatewayPluginCollection.php
+++ /dev/null
@@ -1,62 +0,0 @@
-<?php
-
-namespace Drupal\commerce_payment;
-
-use Drupal\Component\Plugin\Exception\PluginException;
-use Drupal\Component\Plugin\PluginManagerInterface;
-use Drupal\Core\Plugin\DefaultSingleLazyPluginCollection;
-
-/**
- * A collection of payment gateway plugins.
- */
-class PaymentGatewayPluginCollection extends DefaultSingleLazyPluginCollection {
-
-  /**
-   * The payment gateway entity ID this plugin collection belongs to.
-   *
-   * @var string
-   */
-  protected $entityId;
-
-  /**
-   * Constructs a new PaymentGatewayPluginCollection object.
-   *
-   * @param \Drupal\Component\Plugin\PluginManagerInterface $manager
-   *   The manager to be used for instantiating plugins.
-   * @param string $instance_id
-   *   The ID of the plugin instance.
-   * @param array $configuration
-   *   An array of configuration.
-   * @param string $entity_id
-   *   The payment gateway entity ID this plugin collection belongs to.
-   */
-  public function __construct(PluginManagerInterface $manager, $instance_id, array $configuration, $entity_id) {
-    parent::__construct($manager, $instance_id, $configuration);
-
-    $this->entityId = $entity_id;
-  }
-
-  /**
-   * {@inheritdoc}
-   *
-   * @return \Drupal\commerce_payment\Plugin\Commerce\PaymentGateway\PaymentGatewayInterface
-   *   The payment gateway plugin.
-   */
-  public function &get($instance_id) {
-    return parent::get($instance_id);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  protected function initializePlugin($instance_id) {
-    if (!$instance_id) {
-      throw new PluginException("The payment gateway '{$this->entityId}' did not specify a plugin.");
-    }
-
-    $configuration = ['_entity_id' => $this->entityId] + $this->configuration;
-    $plugin = $this->manager->createInstance($instance_id, $configuration);
-    $this->set($instance_id, $plugin);
-  }
-
-}
diff --git a/modules/payment/tests/src/Unit/CreditCardTest.php b/modules/payment/tests/src/Unit/CreditCardTest.php
index b983abcd32..a20f07bf9c 100644
--- a/modules/payment/tests/src/Unit/CreditCardTest.php
+++ b/modules/payment/tests/src/Unit/CreditCardTest.php
@@ -4,12 +4,13 @@
 
 use Drupal\commerce_payment\CreditCard;
 use Drupal\commerce_payment\CreditCardType;
+use Drupal\Tests\UnitTestCase;
 
 /**
  * @coversDefaultClass \Drupal\commerce_payment\CreditCard
  * @group commerce
  */
-class CreditCardTest extends \PHPUnit_Framework_TestCase {
+class CreditCardTest extends UnitTestCase {
 
   /**
    * @covers ::getTypes
diff --git a/modules/tax/src/Entity/TaxType.php b/modules/tax/src/Entity/TaxType.php
index 2d93a7418b..2a09314c3f 100644
--- a/modules/tax/src/Entity/TaxType.php
+++ b/modules/tax/src/Entity/TaxType.php
@@ -2,7 +2,7 @@
 
 namespace Drupal\commerce_tax\Entity;
 
-use Drupal\commerce_tax\TaxTypePluginCollection;
+use Drupal\commerce\CommerceSinglePluginCollection;
 use Drupal\Core\Config\Entity\ConfigEntityBase;
 
 /**
@@ -83,7 +83,7 @@ class TaxType extends ConfigEntityBase implements TaxTypeInterface {
   /**
    * The plugin collection that holds the tax type plugin.
    *
-   * @var \Drupal\commerce_tax\TaxTypePluginCollection
+   * @var \Drupal\commerce\CommerceSinglePluginCollection
    */
   protected $pluginCollection;
 
@@ -157,13 +157,13 @@ public function set($property_name, $value) {
    *
    * Ensures the plugin collection is initialized before returning it.
    *
-   * @return \Drupal\commerce_tax\TaxTypePluginCollection
+   * @return \Drupal\commerce\CommerceSinglePluginCollection
    *   The plugin collection.
    */
   protected function getPluginCollection() {
     if (!$this->pluginCollection) {
       $plugin_manager = \Drupal::service('plugin.manager.commerce_tax_type');
-      $this->pluginCollection = new TaxTypePluginCollection($plugin_manager, $this->plugin, $this->configuration, $this->id);
+      $this->pluginCollection = new CommerceSinglePluginCollection($plugin_manager, $this->plugin, $this->configuration, $this->id);
     }
     return $this->pluginCollection;
   }
diff --git a/modules/tax/src/TaxTypePluginCollection.php b/src/CommerceSinglePluginCollection.php
similarity index 65%
rename from modules/tax/src/TaxTypePluginCollection.php
rename to src/CommerceSinglePluginCollection.php
index 064512237d..463580fa12 100644
--- a/modules/tax/src/TaxTypePluginCollection.php
+++ b/src/CommerceSinglePluginCollection.php
@@ -1,25 +1,25 @@
 <?php
 
-namespace Drupal\commerce_tax;
+namespace Drupal\commerce;
 
 use Drupal\Component\Plugin\Exception\PluginException;
 use Drupal\Component\Plugin\PluginManagerInterface;
 use Drupal\Core\Plugin\DefaultSingleLazyPluginCollection;
 
 /**
- * A collection of tax type plugins.
+ * A collection that stores a single plugin, aware of its parent entity ID.
  */
-class TaxTypePluginCollection extends DefaultSingleLazyPluginCollection {
+class CommerceSinglePluginCollection extends DefaultSingleLazyPluginCollection {
 
   /**
-   * The tax type entity ID this plugin collection belongs to.
+   * The entity ID this plugin collection belongs to.
    *
    * @var string
    */
   protected $entityId;
 
   /**
-   * Constructs a new TaxTypePluginCollection object.
+   * Constructs a new CommerceSinglePluginCollection object.
    *
    * @param \Drupal\Component\Plugin\PluginManagerInterface $manager
    *   The manager to be used for instantiating plugins.
@@ -28,12 +28,13 @@ class TaxTypePluginCollection extends DefaultSingleLazyPluginCollection {
    * @param array $configuration
    *   An array of configuration.
    * @param string $entity_id
-   *   The tax type entity ID this plugin collection belongs to.
+   *   The entity ID this plugin collection belongs to.
    */
   public function __construct(PluginManagerInterface $manager, $instance_id, array $configuration, $entity_id) {
-    parent::__construct($manager, $instance_id, $configuration);
-
     $this->entityId = $entity_id;
+    // The parent constructor initializes the plugin, so it needs to be called
+    // after $this->entityId is set.
+    parent::__construct($manager, $instance_id, $configuration);
   }
 
   /**
@@ -41,7 +42,7 @@ public function __construct(PluginManagerInterface $manager, $instance_id, array
    */
   protected function initializePlugin($instance_id) {
     if (!$instance_id) {
-      throw new PluginException("The tax type '{$this->entityId}' did not specify a plugin.");
+      throw new PluginException("The entity '{$this->entityId}' did not specify a plugin.");
     }
 
     $configuration = ['_entity_id' => $this->entityId] + $this->configuration;