diff --git a/src/MyParcelComApi.php b/src/MyParcelComApi.php
index c2c89fd..ce2f17d 100644
--- a/src/MyParcelComApi.php
+++ b/src/MyParcelComApi.php
@@ -385,7 +385,7 @@ public function resolveDynamicServiceRates(
         ShipmentInterface|array $shipmentData,
         ?ServiceRateInterface $dynamicServiceRate = null,
     ): array {
-        $data = ($shipmentData instanceof ShipmentInterface) ? $shipmentData->jsonSerialize() : $shipmentData;
+        $data = ($shipmentData instanceof ShipmentInterface) ? $shipmentData->getData() : $shipmentData;
 
         if (!isset($data['relationships'])) {
             $data['relationships'] = [];
@@ -584,7 +584,7 @@ public function createAndRegisterShipment(
             '/registered-shipments?' . http_build_query(['include' => Shipment::RELATIONSHIP_FILES]),
             'post',
             [
-                'data' => $shipment,
+                'data' => $shipment->getData(),
                 'meta' => array_filter($shipment->getMeta()),
             ],
             $this->authenticator->getAuthorizationHeader() + [
@@ -648,7 +648,7 @@ public function createAndRegisterMultiColliShipment(
             ]),
             'post',
             [
-                'data' => $shipment,
+                'data' => $shipment->getData(),
                 'meta' => array_merge(
                     [
                         'colli' => array_map(
diff --git a/src/Resources/Proxy/ShipmentProxy.php b/src/Resources/Proxy/ShipmentProxy.php
index 77ea8d4..5000b0c 100644
--- a/src/Resources/Proxy/ShipmentProxy.php
+++ b/src/Resources/Proxy/ShipmentProxy.php
@@ -40,6 +40,11 @@ class ShipmentProxy implements ShipmentInterface, ResourceProxyInterface
 
     private string $type = ResourceInterface::TYPE_SHIPMENT;
 
+    public function getData(): array
+    {
+        return $this->getResource()->getData();
+    }
+
     public function getMeta(): array
     {
         return $this->getResource()->getMeta();
diff --git a/src/Resources/Shipment.php b/src/Resources/Shipment.php
index 24322f9..3e7506d 100644
--- a/src/Resources/Shipment.php
+++ b/src/Resources/Shipment.php
@@ -163,6 +163,22 @@ class Shipment implements ShipmentInterface
     /** @var callable */
     private $statusHistoryCallback = null;
 
+    /**
+     * Prepare the data for a request to our API. This filters the read-only relationships to avoid validation errors.
+     */
+    public function getData(): array
+    {
+        $data = $this->jsonSerialize();
+
+        // Remove read-only relationships.
+        unset($data['relationships']['colli']);
+        unset($data['relationships']['files']);
+        unset($data['relationships']['shipment_status']);
+        unset($data['relationships']['shipment_surcharges']);
+
+        return $data;
+    }
+
     public function getMeta(): array
     {
         return $this->meta;