diff --git a/tests/unit/services/CustomersTest.php b/tests/unit/services/CustomersTest.php
new file mode 100644
index 0000000000..50586f5cff
--- /dev/null
+++ b/tests/unit/services/CustomersTest.php
@@ -0,0 +1,114 @@
+<?php
+/**
+ * @link https://craftcms.com/
+ * @copyright Copyright (c) Pixel & Tonic, Inc.
+ * @license https://craftcms.github.io/license/
+ */
+
+namespace unit\services;
+
+use Codeception\Test\Unit;
+use craft\commerce\elements\Order;
+use craft\commerce\Plugin;
+use craft\commerce\services\Customers;
+use craftcommercetests\fixtures\CustomerFixture;
+use craftcommercetests\fixtures\OrdersFixture;
+use UnitTester;
+
+/**
+ * CustomersTest
+ *
+ * @author Pixel & Tonic, Inc. <support@pixelandtonic.com>
+ * @since 4.3.0
+ */
+class CustomersTest extends Unit
+{
+    /**
+     * @var UnitTester
+     */
+    protected UnitTester $tester;
+
+    /**
+     * @var OrdersFixture
+     */
+    protected OrdersFixture $fixtureData;
+
+    /**
+     * @var array
+     */
+    private array $_deleteElementIds = [];
+
+    /**
+     * @return array
+     */
+    public function _fixtures(): array
+    {
+        return [
+            'customer' => [
+                'class' => CustomerFixture::class,
+            ],
+            'orders' => [
+                'class' => OrdersFixture::class,
+            ],
+        ];
+    }
+
+    protected function _before(): void
+    {
+        parent::_before();
+
+        $this->fixtureData = $this->tester->grabFixture('orders');
+    }
+
+    public function testOrderCompleteHandlerNotCalled(): void
+    {
+        Plugin::getInstance()->set('customers', $this->make(Customers::class, [
+            'orderCompleteHandler' => function() {
+                self::never();
+            },
+        ]));
+
+        /** @var Order $completedOrder */
+        $completedOrder = $this->fixtureData->getElement('completed-new');
+
+        self::assertTrue($completedOrder->markAsComplete());
+    }
+
+    public function testOrderCompleteHandlerCalled(): void
+    {
+        Plugin::getInstance()->set('customers', $this->make(Customers::class, [
+            'orderCompleteHandler' => function() {
+                self::once();
+            },
+        ]));
+
+        $order = new Order();
+        $email = 'test@newemailaddress.xyz';
+        $order->setEmail($email);
+
+        /** @var Order $order */
+        $completedOrder = $this->fixtureData->getElement('completed-new');
+        $lineItem = $completedOrder->getLineItems()[0];
+        $qty = 4;
+        $note = 'My note';
+        $lineItem = Plugin::getInstance()->getLineItems()->createLineItem($order, $lineItem->purchasableId, [], $qty, $note);
+        $order->setLineItems([$lineItem]);
+
+        self::assertTrue($order->markAsComplete());
+
+        $this->_deleteElementIds[] = $order->id;
+    }
+
+    /**
+     * @inheritdoc
+     */
+    protected function _after(): void
+    {
+        parent::_after();
+
+        // Cleanup data.
+        foreach ($this->_deleteElementIds as $elementId) {
+            \Craft::$app->getElements()->deleteElementById($elementId, null, null, true);
+        }
+    }
+}