diff --git a/test/integration/PaymentTest.php b/test/integration/PaymentTest.php
index 037e619c..7b865d20 100755
--- a/test/integration/PaymentTest.php
+++ b/test/integration/PaymentTest.php
@@ -28,6 +28,7 @@
 use heidelpayPHP\Constants\ApiResponseCodes;
 use heidelpayPHP\Exceptions\HeidelpayApiException;
 use heidelpayPHP\Resources\Payment;
+use heidelpayPHP\Resources\PaymentTypes\Card;
 use heidelpayPHP\Resources\PaymentTypes\Paypal;
 use heidelpayPHP\Resources\TransactionTypes\Authorization;
 use heidelpayPHP\Resources\TransactionTypes\Charge;
@@ -196,45 +197,74 @@ public function chargePaymentShouldThrowErrorOnNonPaymentId()
     }
 
     /**
-     * Verify an Exception is thrown if the orderId already exists.
+     * Verify a payment is fetched by orderId if the id is not set.
      *
      * @test
      *
      * @throws HeidelpayApiException A HeidelpayApiException is thrown if there is an error returned on API-request.
      * @throws RuntimeException      A RuntimeException is thrown when there is an error while using the SDK.
      */
-    public function apiShouldReturnErrorIfOrderIdAlreadyExists()
+    public function paymentShouldBeFetchedByOrderIdIfIdIsNotSet()
     {
         $orderId = str_replace(' ', '', microtime());
-
         $paypal = $this->heidelpay->createPaymentType(new Paypal());
         $authorization = $this->heidelpay->authorize(100.00, 'EUR', $paypal, 'http://heidelpay.com', null, $orderId, null, null, false);
-        $this->assertNotEmpty($authorization);
+        $payment = $authorization->getPayment();
+        $fetchedPayment = $this->heidelpay->fetchPaymentByOrderId($orderId);
+
+        $this->assertNotSame($payment, $fetchedPayment);
+        $this->assertEquals($payment->expose(), $fetchedPayment->expose());
+    }
 
-        $paypal2 = $this->heidelpay->createPaymentType(new Paypal());
+    /**
+     * Verify orderId does not need to be unique.
+     *
+     * @test
+     *
+     * @throws HeidelpayApiException
+     * @throws RuntimeException
+     */
+    public function shouldAllowNonUniqueOrderId()
+    {
+        $orderId = self::generateRandomId();
 
-        $this->expectException(HeidelpayApiException::class);
-        $this->expectExceptionCode(ApiResponseCodes::API_ERROR_ORDER_ID_ALREADY_IN_USE);
-        $this->heidelpay->authorize(101.00, 'EUR', $paypal2, 'http://heidelpay.com', null, $orderId, null, null, false);
+        /** @var Card $card */
+        $card = $this->heidelpay->createPaymentType($this->createCardObject());
+        $card->charge(1023, 'EUR', self::RETURN_URL, null, $orderId);
+
+        try {
+            /** @var Card $card2 */
+            $card2 = $this->heidelpay->createPaymentType($this->createCardObject());
+            $card2->charge(1023, 'EUR', self::RETURN_URL, null, $orderId);
+            $this->assertTrue(true);
+        } catch (HeidelpayApiException $e) {
+            $this->assertTrue(false, "No exception expected here. ({$e->getMerchantMessage()})");
+        }
     }
 
     /**
-     * Verify a payment is fetched by orderId if the id is not set.
+     * Verify invoiceId does not need to be unique.
      *
      * @test
      *
-     * @throws HeidelpayApiException A HeidelpayApiException is thrown if there is an error returned on API-request.
-     * @throws RuntimeException      A RuntimeException is thrown when there is an error while using the SDK.
+     * @throws HeidelpayApiException
+     * @throws RuntimeException
      */
-    public function paymentShouldBeFetchedByOrderIdIfIdIsNotSet()
+    public function shouldAllowNonUniqueInvoiceId()
     {
-        $orderId = str_replace(' ', '', microtime());
-        $paypal = $this->heidelpay->createPaymentType(new Paypal());
-        $authorization = $this->heidelpay->authorize(100.00, 'EUR', $paypal, 'http://heidelpay.com', null, $orderId, null, null, false);
-        $payment = $authorization->getPayment();
-        $fetchedPayment = $this->heidelpay->fetchPaymentByOrderId($orderId);
+        $invoiceId = self::generateRandomId();
 
-        $this->assertNotSame($payment, $fetchedPayment);
-        $this->assertEquals($payment->expose(), $fetchedPayment->expose());
+        /** @var Card $card */
+        $card = $this->heidelpay->createPaymentType($this->createCardObject());
+        $card->charge(1023, 'EUR', self::RETURN_URL, null, null, null, null, null, $invoiceId);
+
+        try {
+            /** @var Card $card2 */
+            $card2 = $this->heidelpay->createPaymentType($this->createCardObject());
+            $card2->charge(1023, 'EUR', self::RETURN_URL, null, null, null, null, null, $invoiceId);
+            $this->assertTrue(true);
+        } catch (HeidelpayApiException $e) {
+            $this->assertTrue(false, "No exception expected here. ({$e->getMerchantMessage()})");
+        }
     }
 }