diff -u b/modules/promotion/src/Plugin/Commerce/PromotionOffer/BuyXGetY.php b/modules/promotion/src/Plugin/Commerce/PromotionOffer/BuyXGetY.php
--- b/modules/promotion/src/Plugin/Commerce/PromotionOffer/BuyXGetY.php
+++ b/modules/promotion/src/Plugin/Commerce/PromotionOffer/BuyXGetY.php
@@ -89,8 +89,7 @@
       $plugin_definition,
       $container->get('commerce_price.rounder'),
       $container->get('commerce_order.price_splitter'),
-      $container->get('plugin.manager.commerce_condition'),
-      $container->get('commerce_cart.cart_manager')
+      $container->get('plugin.manager.commerce_condition')
     );
   }
 
@@ -269,7 +268,6 @@
     }
 
     $get_conditions = $this->buildConditionGroup($this->configuration['get_conditions']);
-
     // @todo Make this behavior optional based on an additional promotion
     //   setting.
     if ($get_purchasable_entity = $this->findSinglePurchasableEntity($get_conditions)) {
@@ -410,9 +408,13 @@
   }
 
   /**
+   * Find the configured purchasable entity amongst the given conditions.
+   *
    * @param \Drupal\commerce\ConditionGroup $conditions
+   *   The condition group.
    *
    * @return \Drupal\commerce\PurchasableEntityInterface|null
+   *   The purchasable entity, NULL if not found in the conditions.
    */
   protected function findSinglePurchasableEntity(ConditionGroup $conditions) {
     foreach ($conditions->getConditions() as $condition) {
@@ -429,10 +431,18 @@
   }
 
   /**
+   * Attempt to find the given purchasable entity amongst the given order items.
+   *
+   * If the given purchasable entity isn't referenced by any order item, create
+   * an order item referencing it so we can automatically add it to the order.
+   *
    * @param \Drupal\commerce\PurchasableEntityInterface $get_purchasable_entity
+   *   The "get" purchasable entity.
    * @param \Drupal\commerce_order\Entity\OrderItemInterface[] $order_items
+   *   The order items.
    *
-   * @return \Drupal\commerce_order\Entity\OrderItemInterface|null
+   * @return \Drupal\commerce_order\Entity\OrderItemInterface
+   *   An order item referencing the given purchasable entity.
    */
   protected function findOrCreateOrderItem(PurchasableEntityInterface $get_purchasable_entity, array $order_items) {
     foreach ($order_items as $order_item) {
@@ -452,10 +462,15 @@
   }
 
   /**
+   * Calculates the expected get quantity.
+   *
    * @param array $buy_quantities
+   *   An array of buy quantities.
    * @param \Drupal\commerce_order\Entity\OrderItemInterface $order_item
+   *   The order item.
    *
    * @return string
+   *   The expected get quantity.
    */
   protected function calculateExpectedGetQuantity($buy_quantities, OrderItemInterface $order_item) {
     $expected_get_quantity = '0';