diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 39068384..f66ce864 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Composer
         uses: php-actions/composer@v6
         with:
@@ -20,7 +20,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Composer
         uses: php-actions/composer@v6
         with:
@@ -35,7 +35,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Composer
         uses: php-actions/composer@v6
         with:
@@ -50,7 +50,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Composer
         uses: php-actions/composer@v6
         with:
@@ -61,3 +61,18 @@ jobs:
           version: 9
           php_version: 8.3
           configuration: phpunit.xml
+  php84:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+      - name: Composer
+        uses: php-actions/composer@v6
+        with:
+          php_version: 8.4
+      - name: PHPUnit
+        uses: php-actions/phpunit@v4
+        with:
+          version: 9
+          php_version: 8.4
+          configuration: phpunit.xml
diff --git a/src/MyParcelComApi.php b/src/MyParcelComApi.php
index 9003daa9..f49b5036 100644
--- a/src/MyParcelComApi.php
+++ b/src/MyParcelComApi.php
@@ -69,9 +69,9 @@ class MyParcelComApi implements MyParcelComApiInterface
     public static function createSingleton(
         AuthenticatorInterface $authenticator,
         string $apiUri = 'https://api.sandbox.myparcel.com',
-        ClientInterface $httpClient = null,
-        CacheInterface $cache = null,
-        ResourceFactoryInterface $resourceFactory = null,
+        ?ClientInterface $httpClient = null,
+        ?CacheInterface $cache = null,
+        ?ResourceFactoryInterface $resourceFactory = null,
     ): self {
         return self::$singleton = (new self($apiUri, $httpClient, $cache, $resourceFactory))
             ->authenticate($authenticator);
@@ -91,9 +91,9 @@ public static function getSingleton(): ?self
      */
     public function __construct(
         string $apiUri = 'https://api.sandbox.myparcel.com',
-        ClientInterface $httpClient = null,
-        CacheInterface $cache = null,
-        ResourceFactoryInterface $resourceFactory = null,
+        ?ClientInterface $httpClient = null,
+        ?CacheInterface $cache = null,
+        ?ResourceFactoryInterface $resourceFactory = null,
     ) {
         if ($httpClient === null) {
             $httpClient = HttpClientDiscovery::find();
@@ -154,7 +154,7 @@ public function getPickUpDropOffLocations(
         string $postalCode,
         ?string $streetName = null,
         ?string $streetNumber = null,
-        CarrierInterface $specificCarrier = null,
+        ?CarrierInterface $specificCarrier = null,
         bool $onlyActiveContracts = true,
         int $ttl = self::TTL_10MIN,
         ?array $filters = null,
@@ -242,7 +242,7 @@ public function getDefaultShop(int $ttl = self::TTL_10MIN): ShopInterface
     }
 
     public function getServices(
-        ShipmentInterface $shipment = null,
+        ?ShipmentInterface $shipment = null,
         array $filters = ['has_active_contract' => 'true'],
         int $ttl = self::TTL_10MIN,
     ): ResourceCollectionInterface {
@@ -453,8 +453,10 @@ public function resolveDynamicServiceRates(
         return $this->jsonToResources($json['data'], $included);
     }
 
-    public function getShipments(ShopInterface $shop = null, int $ttl = self::TTL_NO_CACHE): ResourceCollectionInterface
-    {
+    public function getShipments(
+        ?ShopInterface $shop = null,
+        int $ttl = self::TTL_NO_CACHE
+    ): ResourceCollectionInterface {
         $url = new UrlBuilder($this->apiUri . self::PATH_SHIPMENTS);
 
         $url->addQuery([
@@ -1337,7 +1339,7 @@ protected function sendResource(
         return reset($resources);
     }
 
-    protected function getResourceUri(string $resourceType, string $id = null): string
+    protected function getResourceUri(string $resourceType, ?string $id = null): string
     {
         return implode(
             '/',
@@ -1384,7 +1386,7 @@ private function arrayToFilter(array &$filters, array $keys, mixed $value): void
      */
     private function determineCarriersForPudoLocations(
         bool $onlyActiveContracts,
-        CarrierInterface $specificCarrier = null,
+        ?CarrierInterface $specificCarrier = null,
     ): array {
         // If we're looking for a specific carrier but it doesn't
         // matter if it has active contracts, just return it immediately.
diff --git a/src/MyParcelComApiInterface.php b/src/MyParcelComApiInterface.php
index a48b50be..c7fc5532 100644
--- a/src/MyParcelComApiInterface.php
+++ b/src/MyParcelComApiInterface.php
@@ -78,7 +78,7 @@ public function getPickUpDropOffLocations(
         string $postalCode,
         ?string $streetName = null,
         ?string $streetNumber = null,
-        CarrierInterface $specificCarrier = null,
+        ?CarrierInterface $specificCarrier = null,
         bool $onlyActiveContracts = true,
         int $ttl = self::TTL_10MIN,
         ?array $filters = null,
@@ -106,7 +106,7 @@ public function getDefaultShop(int $ttl = self::TTL_10MIN): ShopInterface;
      * @throws MyParcelComException
      */
     public function getServices(
-        ShipmentInterface $shipment = null,
+        ?ShipmentInterface $shipment = null,
         array $filters = ['has_active_contract' => 'true'],
         int $ttl = self::TTL_10MIN,
     ): ResourceCollectionInterface;
@@ -158,7 +158,7 @@ public function resolveDynamicServiceRates(
      * @throws MyParcelComException
      */
     public function getShipments(
-        ShopInterface $shop = null,
+        ?ShopInterface $shop = null,
         int $ttl = self::TTL_NO_CACHE,
     ): ResourceCollectionInterface;
 
diff --git a/src/Resources/Interfaces/ShipmentInterface.php b/src/Resources/Interfaces/ShipmentInterface.php
index eb1c4a8c..b1f7e41a 100644
--- a/src/Resources/Interfaces/ShipmentInterface.php
+++ b/src/Resources/Interfaces/ShipmentInterface.php
@@ -158,7 +158,7 @@ public function addFile(FileInterface $file): self;
     /**
      * @return FileInterface[]
      */
-    public function getFiles(string $type = null): array;
+    public function getFiles(?string $type = null): array;
 
     public function setShipmentStatus(ShipmentStatusInterface $status): self;
 
diff --git a/src/Resources/Proxy/ShipmentProxy.php b/src/Resources/Proxy/ShipmentProxy.php
index 5000b0c7..95b4c581 100644
--- a/src/Resources/Proxy/ShipmentProxy.php
+++ b/src/Resources/Proxy/ShipmentProxy.php
@@ -390,7 +390,7 @@ public function addFile(FileInterface $file): self
         return $this;
     }
 
-    public function getFiles(string $type = null): array
+    public function getFiles(?string $type = null): array
     {
         return $this->getResource()->getFiles($type);
     }
diff --git a/src/Resources/Service.php b/src/Resources/Service.php
index 8ad4499c..215c65fe 100644
--- a/src/Resources/Service.php
+++ b/src/Resources/Service.php
@@ -7,6 +7,7 @@
 use MyParcelCom\ApiSdk\Resources\Interfaces\CarrierInterface;
 use MyParcelCom\ApiSdk\Resources\Interfaces\ResourceInterface;
 use MyParcelCom\ApiSdk\Resources\Interfaces\ServiceInterface;
+use MyParcelCom\ApiSdk\Resources\Interfaces\ServiceOptionInterface;
 use MyParcelCom\ApiSdk\Resources\Interfaces\ServiceRateInterface;
 use MyParcelCom\ApiSdk\Resources\Traits\JsonSerializable;
 use MyParcelCom\ApiSdk\Resources\Traits\ProcessIncludes;
@@ -33,9 +34,11 @@ class Service implements ServiceInterface
     const ATTRIBUTE_VOLUMETRIC_WEIGHT_DIVISOR = 'volumetric_weight_divisor';
 
     const RELATIONSHIP_CARRIER = 'carrier';
+    const RELATIONSHIP_SERVICE_OPTIONS = 'service_options';
 
     const INCLUDES = [
         ResourceInterface::TYPE_CARRIER => self::RELATIONSHIP_CARRIER,
+        ResourceInterface::TYPE_SERVICE_OPTION => self::RELATIONSHIP_SERVICE_OPTIONS,
     ];
 
     private ?string $id = null;
@@ -63,6 +66,9 @@ class Service implements ServiceInterface
         self::RELATIONSHIP_CARRIER => [
             'data' => null,
         ],
+        self::RELATIONSHIP_SERVICE_OPTIONS => [
+            'data' => [],
+        ],
     ];
 
     /** @var ServiceRateInterface[] */
@@ -143,6 +149,29 @@ public function getCarrier(): CarrierInterface
         return $this->relationships[self::RELATIONSHIP_CARRIER]['data'];
     }
 
+    public function setServiceOptions(array $serviceOptions): self
+    {
+        $this->relationships[self::RELATIONSHIP_SERVICE_OPTIONS]['data'] = [];
+
+        foreach ($serviceOptions as $serviceOption) {
+            $this->addServiceOption($serviceOption);
+        }
+
+        return $this;
+    }
+
+    public function addServiceOption(ServiceOptionInterface $serviceOption): self
+    {
+        $this->relationships[self::RELATIONSHIP_SERVICE_OPTIONS]['data'][] = $serviceOption;
+
+        return $this;
+    }
+
+    public function getServiceOptions(): array
+    {
+        return $this->relationships[self::RELATIONSHIP_SERVICE_OPTIONS]['data'];
+    }
+
     public function setHandoverMethod(string $handoverMethod): self
     {
         $this->attributes[self::ATTRIBUTE_HANDOVER_METHOD] = $handoverMethod;
diff --git a/src/Resources/Shipment.php b/src/Resources/Shipment.php
index 7b70bea0..dd3f73a4 100644
--- a/src/Resources/Shipment.php
+++ b/src/Resources/Shipment.php
@@ -591,7 +591,7 @@ public function addFile(FileInterface $file): self
         return $this;
     }
 
-    public function getFiles(string $type = null): array
+    public function getFiles(?string $type = null): array
     {
         // For multi-colli `master` shipments we make this function return all files from the related `colli` shipments.
         if (!empty($this->getColli())) {
diff --git a/src/Utils/UrlBuilder.php b/src/Utils/UrlBuilder.php
index aa9c33f0..d7af9ba6 100644
--- a/src/Utils/UrlBuilder.php
+++ b/src/Utils/UrlBuilder.php
@@ -16,7 +16,7 @@ class UrlBuilder
     protected ?string $fragment = null;
     protected array $query = [];
 
-    public function __construct(string $url = null)
+    public function __construct(?string $url = null)
     {
         if ($url !== null) {
             $this->setUrl($url);
diff --git a/tests/Feature/ResourceFactoryTest.php b/tests/Feature/ResourceFactoryTest.php
index d2d85698..0281f676 100644
--- a/tests/Feature/ResourceFactoryTest.php
+++ b/tests/Feature/ResourceFactoryTest.php
@@ -304,12 +304,15 @@ public function testCreateService()
                 'uses_volumetric_weight' => true,
             ],
             'relationships' => [
-                'carrier' => [
+                'carrier'         => [
                     'data' => [
                         'id'   => 'carrier-id-1',
                         'type' => 'carriers',
                     ],
                 ],
+                'service_options' => [
+                    'data' => [],
+                ],
             ],
         ];
 
diff --git a/tests/Unit/Resources/ServiceTest.php b/tests/Unit/Resources/ServiceTest.php
index f867415b..bbf15e03 100644
--- a/tests/Unit/Resources/ServiceTest.php
+++ b/tests/Unit/Resources/ServiceTest.php
@@ -37,7 +37,10 @@ public function testName()
     public function testPackageType()
     {
         $service = new Service();
-        $this->assertEquals(Service::PACKAGE_TYPE_PARCEL, $service->setPackageType(Service::PACKAGE_TYPE_PARCEL)->getPackageType());
+        $this->assertEquals(
+            Service::PACKAGE_TYPE_PARCEL,
+            $service->setPackageType(Service::PACKAGE_TYPE_PARCEL)->getPackageType(),
+        );
     }
 
     /** @test */
@@ -199,12 +202,15 @@ public function testJsonSerialize()
                 'uses_volumetric_weight' => true,
             ],
             'relationships' => [
-                'carrier' => [
+                'carrier'         => [
                     'data' => [
                         'id'   => 'carrier-id-1',
                         'type' => 'carriers',
                     ],
                 ],
+                'service_options' => [
+                    'data' => [],
+                ],
             ],
         ], $service->jsonSerialize());
     }