diff --git a/backend/app/Listeners/Order/SendOrderDetailsEmailListener.php b/backend/app/Listeners/Order/SendOrderDetailsEmailListener.php
index 5cfa051d..85ee2256 100644
--- a/backend/app/Listeners/Order/SendOrderDetailsEmailListener.php
+++ b/backend/app/Listeners/Order/SendOrderDetailsEmailListener.php
@@ -5,7 +5,7 @@
 use HiEvents\Events\OrderStatusChangedEvent;
 use HiEvents\Jobs\Order\SendOrderDetailsEmailJob;
 
-readonly class SendOrderDetailsEmailListener
+class SendOrderDetailsEmailListener
 {
     public function handle(OrderStatusChangedEvent $changedEvent): void
     {
diff --git a/backend/app/Services/Handlers/Order/CompleteOrderHandler.php b/backend/app/Services/Handlers/Order/CompleteOrderHandler.php
index 33a18197..0d9751cf 100644
--- a/backend/app/Services/Handlers/Order/CompleteOrderHandler.php
+++ b/backend/app/Services/Handlers/Order/CompleteOrderHandler.php
@@ -15,10 +15,12 @@
 use HiEvents\DomainObjects\Status\AttendeeStatus;
 use HiEvents\DomainObjects\Status\OrderPaymentStatus;
 use HiEvents\DomainObjects\Status\OrderStatus;
+use HiEvents\DomainObjects\TicketDomainObject;
 use HiEvents\DomainObjects\TicketPriceDomainObject;
 use HiEvents\Events\OrderStatusChangedEvent;
 use HiEvents\Exceptions\ResourceConflictException;
 use HiEvents\Helper\IdHelper;
+use HiEvents\Repository\Eloquent\Value\Relationship;
 use HiEvents\Repository\Interfaces\AttendeeRepositoryInterface;
 use HiEvents\Repository\Interfaces\OrderRepositoryInterface;
 use HiEvents\Repository\Interfaces\QuestionAnswerRepositoryInterface;
@@ -37,14 +39,14 @@
 /**
  * @todo - Tidy this up
  */
-readonly class CompleteOrderHandler
+class CompleteOrderHandler
 {
     public function __construct(
-        private OrderRepositoryInterface          $orderRepository,
-        private AttendeeRepositoryInterface       $attendeeRepository,
-        private QuestionAnswerRepositoryInterface $questionAnswersRepository,
-        private TicketQuantityUpdateService       $ticketQuantityUpdateService,
-        private TicketPriceRepositoryInterface    $ticketPriceRepository,
+        private readonly OrderRepositoryInterface          $orderRepository,
+        private readonly AttendeeRepositoryInterface       $attendeeRepository,
+        private readonly QuestionAnswerRepositoryInterface $questionAnswersRepository,
+        private readonly TicketQuantityUpdateService       $ticketQuantityUpdateService,
+        private readonly TicketPriceRepositoryInterface    $ticketPriceRepository,
     )
     {
     }
@@ -89,7 +91,6 @@ public function handle(string $orderShortId, CompleteOrderDTO $orderData): Order
     private function createAttendees(Collection $attendees, OrderDomainObject $order): void
     {
         $inserts = [];
-        $publicIdIndex = 1;
 
         $ticketsPrices = $this->ticketPriceRepository->findWhereIn(
             field: TicketPriceDomainObjectAbstract::ID,
@@ -97,6 +98,7 @@ private function createAttendees(Collection $attendees, OrderDomainObject $order
         );
 
         $this->validateTicketPriceIdsMatchOrder($order, $ticketsPrices);
+        $this->validateAttendees($order, $attendees);
 
         foreach ($attendees as $attendee) {
             $ticketId = $ticketsPrices->first(
@@ -192,7 +194,7 @@ private function createAttendeeQuestions(
     private function validateOrder(OrderDomainObject $order): void
     {
         if ($order->getEmail() !== null) {
-            throw new ResourceConflictException(__('This order is has already been processed'));
+            throw new ResourceConflictException(__('This order has already been processed'));
         }
 
         if (Carbon::createFromTimeString($order->getReservedUntil())->isPast()) {
@@ -210,7 +212,11 @@ private function validateOrder(OrderDomainObject $order): void
     private function getOrder(string $orderShortId): OrderDomainObject
     {
         $order = $this->orderRepository
-            ->loadRelation(OrderItemDomainObject::class)
+            ->loadRelation(
+                new Relationship(
+                    domainObject: OrderItemDomainObject::class,
+                    nested: [new Relationship(TicketDomainObject::class, name: 'ticket')]
+                ))
             ->findByShortId($orderShortId);
 
         if ($order === null) {
@@ -258,4 +264,18 @@ private function validateTicketPriceIdsMatchOrder(OrderDomainObject $order, Coll
             throw new ResourceConflictException(__('There is an unexpected ticket price ID in the order'));
         }
     }
+
+    /**
+     * @throws ResourceConflictException
+     */
+    private function validateAttendees(OrderDomainObject $order, Collection $attendees): void
+    {
+        $orderAttendeeCount = $order->getOrderItems()->sum(fn(OrderItemDomainObject $orderItem) => $orderItem->getQuantity());
+
+        if ($orderAttendeeCount !== $attendees->count()) {
+            throw new ResourceConflictException(
+                __('The number of attendees does not match the number of tickets in the order')
+            );
+        }
+    }
 }
diff --git a/backend/phpunit.xml b/backend/phpunit.xml
index 4f3d5a61..9b11db4f 100644
--- a/backend/phpunit.xml
+++ b/backend/phpunit.xml
@@ -21,7 +21,7 @@
         <env name="APP_ENV" value="testing"/>
         <env name="BCRYPT_ROUNDS" value="4"/>
         <env name="CACHE_DRIVER" value="array"/>
-        <env name="DB_DATABASE" value="testing"/>
+<!--        <env name="DB_DATABASE" value="testing"/>-->
         <env name="MAIL_MAILER" value="array"/>
         <env name="QUEUE_CONNECTION" value="sync"/>
         <env name="SESSION_DRIVER" value="array"/>
diff --git a/backend/tests/Unit/Services/Handlers/Order/CompleteOrderHandlerTest.php b/backend/tests/Unit/Services/Handlers/Order/CompleteOrderHandlerTest.php
new file mode 100644
index 00000000..a68f595e
--- /dev/null
+++ b/backend/tests/Unit/Services/Handlers/Order/CompleteOrderHandlerTest.php
@@ -0,0 +1,299 @@
+<?php
+
+namespace Tests\Unit\Services\Handlers\Order;
+
+use Carbon\Carbon;
+use Exception;
+use HiEvents\DomainObjects\AttendeeDomainObject;
+use HiEvents\DomainObjects\OrderDomainObject;
+use HiEvents\DomainObjects\OrderItemDomainObject;
+use HiEvents\DomainObjects\Status\OrderStatus;
+use HiEvents\DomainObjects\TicketPriceDomainObject;
+use HiEvents\Exceptions\ResourceConflictException;
+use HiEvents\Repository\Interfaces\AttendeeRepositoryInterface;
+use HiEvents\Repository\Interfaces\OrderRepositoryInterface;
+use HiEvents\Repository\Interfaces\QuestionAnswerRepositoryInterface;
+use HiEvents\Repository\Interfaces\TicketPriceRepositoryInterface;
+use HiEvents\Services\Domain\Ticket\TicketQuantityUpdateService;
+use HiEvents\Services\Handlers\Order\CompleteOrderHandler;
+use HiEvents\Services\Handlers\Order\DTO\CompleteOrderAttendeeDTO;
+use HiEvents\Services\Handlers\Order\DTO\CompleteOrderDTO;
+use HiEvents\Services\Handlers\Order\DTO\CompleteOrderOrderDTO;
+use Illuminate\Support\Collection;
+use Illuminate\Support\Facades\Bus;
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Mail;
+use Illuminate\Support\Facades\Queue;
+use Mockery;
+use Mockery\MockInterface;
+use Symfony\Component\Routing\Exception\ResourceNotFoundException;
+use Tests\TestCase;
+use Illuminate\Database\Connection;
+
+class CompleteOrderHandlerTest extends TestCase
+{
+    private OrderRepositoryInterface|MockInterface $orderRepository;
+    private AttendeeRepositoryInterface|MockInterface $attendeeRepository;
+    private QuestionAnswerRepositoryInterface|MockInterface $questionAnswersRepository;
+    private TicketQuantityUpdateService|MockInterface $ticketQuantityUpdateService;
+    private TicketPriceRepositoryInterface|MockInterface $ticketPriceRepository;
+    private CompleteOrderHandler $completeOrderHandler;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+        Queue::fake();
+        Mail::fake();
+        Bus::fake();
+        DB::shouldReceive('transaction')->andReturnUsing(fn($callback) => $callback(Mockery::mock(Connection::class)));
+
+        $this->orderRepository = Mockery::mock(OrderRepositoryInterface::class);
+        $this->attendeeRepository = Mockery::mock(AttendeeRepositoryInterface::class);
+        $this->questionAnswersRepository = Mockery::mock(QuestionAnswerRepositoryInterface::class);
+        $this->ticketQuantityUpdateService = Mockery::mock(TicketQuantityUpdateService::class);
+        $this->ticketPriceRepository = Mockery::mock(TicketPriceRepositoryInterface::class);
+
+        $this->completeOrderHandler = new CompleteOrderHandler(
+            $this->orderRepository,
+            $this->attendeeRepository,
+            $this->questionAnswersRepository,
+            $this->ticketQuantityUpdateService,
+            $this->ticketPriceRepository
+        );
+    }
+
+    protected function tearDown(): void
+    {
+        Mockery::close();
+        parent::tearDown();
+    }
+
+    public function testHandleSuccessfullyCompletesOrder(): void
+    {
+        $orderShortId = 'ABC123';
+        $orderData = $this->createMockCompleteOrderDTO();
+        $order = $this->createMockOrder();
+        $updatedOrder = $this->createMockOrder();
+
+        $this->orderRepository->shouldReceive('findByShortId')->with($orderShortId)->andReturn($order);
+        $this->orderRepository->shouldReceive('loadRelation')->andReturnSelf();
+        $this->orderRepository->shouldReceive('updateFromArray')->andReturn($updatedOrder);
+        $this->orderRepository->shouldReceive('loadRelation')->andReturnSelf();
+
+        $this->ticketPriceRepository->shouldReceive('findWhereIn')->andReturn(new Collection([$this->createMockTicketPrice()]));
+
+        $this->attendeeRepository->shouldReceive('insert')->andReturn(true);
+        $this->attendeeRepository->shouldReceive('findWhere')->andReturn(new Collection([$this->createMockAttendee()]));
+
+        $this->ticketQuantityUpdateService->shouldReceive('updateQuantitiesFromOrder');
+
+        $this->completeOrderHandler->handle($orderShortId, $orderData);
+
+        $this->assertTrue(true);
+    }
+
+    public function testHandleThrowsResourceNotFoundExceptionWhenOrderNotFound(): void
+    {
+        $this->expectException(ResourceNotFoundException::class);
+
+        $orderShortId = 'NONEXISTENT';
+        $orderData = $this->createMockCompleteOrderDTO();
+
+        $this->orderRepository->shouldReceive('findByShortId')->with($orderShortId)->andReturnNull();
+        $this->orderRepository->shouldReceive('loadRelation')->andReturnSelf();
+
+        $this->completeOrderHandler->handle($orderShortId, $orderData);
+    }
+
+    public function testHandleThrowsResourceConflictExceptionWhenOrderAlreadyProcessed(): void
+    {
+        $this->expectException(ResourceConflictException::class);
+        $this->expectExceptionMessage('This order has already been processed');
+
+        $orderShortId = 'ABC123';
+        $orderData = $this->createMockCompleteOrderDTO();
+
+        $order = $this->createMockOrder(OrderStatus::COMPLETED);
+        $order->setEmail('d@d.com');
+        $order->setTotalGross(0);
+
+        $this->orderRepository->shouldReceive('findByShortId')->with($orderShortId)->andReturn($order);
+        $this->orderRepository->shouldReceive('loadRelation')->andReturnSelf();
+
+        $this->completeOrderHandler->handle($orderShortId, $orderData);
+    }
+
+    public function testHandleThrowsResourceConflictExceptionWhenOrderExpired(): void
+    {
+        $this->expectException(ResourceConflictException::class);
+
+        $orderShortId = 'ABC123';
+        $orderData = $this->createMockCompleteOrderDTO();
+        $order = $this->createMockOrder();
+        $order->setEmail('d@d.com');
+        $order->setReservedUntil(Carbon::now()->subHour()->toDateTimeString());
+        $order->setTotalGross(100);
+
+        $this->orderRepository->shouldReceive('findByShortId')->with($orderShortId)->andReturn($order);
+        $this->orderRepository->shouldReceive('loadRelation')->andReturnSelf();
+
+        $this->completeOrderHandler->handle($orderShortId, $orderData);
+    }
+
+    public function testHandleUpdatesTicketQuantitiesForFreeOrder(): void
+    {
+        $orderShortId = 'ABC123';
+        $orderData = $this->createMockCompleteOrderDTO();
+        $order = $this->createMockOrder();
+        $updatedOrder = $this->createMockOrder(OrderStatus::COMPLETED);
+
+        $order->setTotalGross(0);
+        $this->orderRepository->shouldReceive('findByShortId')->with($orderShortId)->andReturn($order);
+        $this->orderRepository->shouldReceive('loadRelation')->andReturnSelf();
+        $this->orderRepository->shouldReceive('updateFromArray')->andReturn($updatedOrder);
+
+        $this->ticketPriceRepository->shouldReceive('findWhereIn')->andReturn(new Collection([$this->createMockTicketPrice()]));
+
+        $this->attendeeRepository->shouldReceive('insert')->andReturn(true);
+        $this->attendeeRepository->shouldReceive('findWhere')->andReturn(new Collection([$this->createMockAttendee()]));
+
+        $this->ticketQuantityUpdateService->shouldReceive('updateQuantitiesFromOrder')->once();
+
+        $order = $this->completeOrderHandler->handle($orderShortId, $orderData);
+
+        $this->assertSame($order->getStatus(), OrderStatus::COMPLETED->name);
+    }
+
+    public function testHandleDoesNotUpdateTicketQuantitiesForPaidOrder(): void
+    {
+        $orderShortId = 'ABC123';
+        $orderData = $this->createMockCompleteOrderDTO();
+        $order = $this->createMockOrder();
+        $updatedOrder = $this->createMockOrder();
+
+        $order->setTotalGross(10);
+
+        $this->orderRepository->shouldReceive('findByShortId')->with($orderShortId)->andReturn($order);
+        $this->orderRepository->shouldReceive('loadRelation')->andReturnSelf();
+        $this->orderRepository->shouldReceive('updateFromArray')->andReturn($updatedOrder);
+
+        $this->ticketPriceRepository->shouldReceive('findWhereIn')->andReturn(new Collection([$this->createMockTicketPrice()]));
+
+        $this->attendeeRepository->shouldReceive('insert')->andReturn(true);
+        $this->attendeeRepository->shouldReceive('findWhere')->andReturn(new Collection([$this->createMockAttendee()]));
+
+        $this->ticketQuantityUpdateService->shouldNotReceive('updateQuantitiesFromOrder');
+
+        $this->completeOrderHandler->handle($orderShortId, $orderData);
+
+        $this->expectNotToPerformAssertions();
+    }
+
+    public function testHandleThrowsExceptionWhenAttendeeInsertFails(): void
+    {
+        $this->expectException(Exception::class);
+
+        $orderShortId = 'ABC123';
+        $orderData = $this->createMockCompleteOrderDTO();
+        $order = $this->createMockOrder();
+        $updatedOrder = $this->createMockOrder();
+
+        $this->orderRepository->shouldReceive('findByShortId')->with($orderShortId)->andReturn($order);
+        $this->orderRepository->shouldReceive('loadRelation')->andReturnSelf();
+        $this->orderRepository->shouldReceive('updateFromArray')->andReturn($updatedOrder);
+
+        $this->ticketPriceRepository->shouldReceive('findWhereIn')->andReturn(new Collection([$this->createMockTicketPrice()]));
+
+        $this->attendeeRepository->shouldReceive('insert')->andReturn(false);
+
+        $this->completeOrderHandler->handle($orderShortId, $orderData);
+    }
+
+    public function testExceptionIsThrowWhenAttendeeCountDoesNotMatchOrderItemsCount(): void
+    {
+        $this->expectException(ResourceConflictException::class);
+        $this->expectExceptionMessage('The number of attendees does not match the number of tickets in the order');
+
+        $orderShortId = 'ABC123';
+        $orderData = $this->createMockCompleteOrderDTO();
+        $order = $this->createMockOrder();
+        $updatedOrder = $this->createMockOrder();
+
+        $order->getOrderItems()->first()->setQuantity(2);
+
+        $this->orderRepository->shouldReceive('findByShortId')->with($orderShortId)->andReturn($order);
+        $this->orderRepository->shouldReceive('loadRelation')->andReturnSelf();
+        $this->orderRepository->shouldReceive('updateFromArray')->andReturn($updatedOrder);
+
+        $this->ticketPriceRepository->shouldReceive('findWhereIn')->andReturn(new Collection([$this->createMockTicketPrice()]));
+
+        $this->attendeeRepository->shouldReceive('insert')->andReturn(true);
+        $this->attendeeRepository->shouldReceive('findWhere')->andReturn(new Collection());
+
+        $this->completeOrderHandler->handle($orderShortId, $orderData);
+    }
+
+    private function createMockCompleteOrderDTO(): CompleteOrderDTO
+    {
+        $orderDTO = new CompleteOrderOrderDTO(
+            first_name: 'John',
+            last_name: 'Doe',
+            email: 'john@example.com',
+            questions: null,
+        );
+
+        $attendeeDTO = new CompleteOrderAttendeeDTO(
+            first_name: 'John',
+            last_name: 'Doe',
+            email: 'john@example.com',
+            ticket_price_id: 1
+        );
+
+        return new CompleteOrderDTO(
+            order: $orderDTO,
+            attendees: new Collection([$attendeeDTO])
+        );
+    }
+
+    private function createMockOrder(OrderStatus $status = OrderStatus::RESERVED): OrderDomainObject|MockInterface
+    {
+        return (new OrderDomainObject())
+            ->setEmail(null)
+            ->setReservedUntil(Carbon::now()->addHour()->toDateTimeString())
+            ->setStatus($status->name)
+            ->setId(1)
+            ->setEventId(1)
+            ->setLocale('en')
+            ->setTotalGross(10)
+            ->setOrderItems(new Collection([
+                $this->createMockOrderItem()
+            ]));
+    }
+
+    private function createMockOrderItem(): OrderItemDomainObject|MockInterface
+    {
+        return (new OrderItemDomainObject())
+            ->setId(1)
+            ->setTicketId(1)
+            ->setQuantity(1)
+            ->setPrice(10)
+            ->setTotalGross(10)
+            ->setTicketPriceId(1);
+    }
+
+    private function createMockTicketPrice(): TicketPriceDomainObject|MockInterface
+    {
+        $ticketPrice = Mockery::mock(TicketPriceDomainObject::class);
+        $ticketPrice->shouldReceive('getId')->andReturn(1);
+        $ticketPrice->shouldReceive('getTicketId')->andReturn(1);
+        return $ticketPrice;
+    }
+
+    private function createMockAttendee(): AttendeeDomainObject|MockInterface
+    {
+        $attendee = Mockery::mock(AttendeeDomainObject::class);
+        $attendee->shouldReceive('getId')->andReturn(1);
+        $attendee->shouldReceive('getTicketId')->andReturn(1);
+        return $attendee;
+    }
+}