diff --git a/tests/src/Functional/PaymentAdminTest.php b/tests/src/Functional/PaymentAdminTest.php
index 79049bd..542f202 100644
--- a/tests/src/Functional/PaymentAdminTest.php
+++ b/tests/src/Functional/PaymentAdminTest.php
@@ -29,13 +29,15 @@ protected function getAdministratorPermissions() {
   }
 
   /**
-   * Tests creating a payment gateway.
+   * Tests creating a payment gateway through the admin UI.
    */
   public function testPaymentGatewayCreation() {
+    // Go to the admin UI and start adding a new payment gateway.
     $this->drupalGet('admin/commerce/config/payment-gateways');
     $this->getSession()->getPage()->clickLink('Add payment gateway');
     $this->assertSession()->addressEquals('admin/commerce/config/payment-gateways/add');
 
+    // Create an array of values to be posted to the add form.
     $values = [
       'label' => 'SIPS',
       'plugin' => 'sips_payment',
@@ -47,23 +49,47 @@ public function testPaymentGatewayCreation() {
       'configuration[sips_key_version]' => 'owl',
     ];
     $this->submitForm($values, 'Save');
+
+    // Check that we ended up on the overview.
     $this->assertSession()->addressEquals('admin/commerce/config/payment-gateways');
     $this->assertSession()->responseContains('SIPS');
     $this->assertSession()->responseContains('Test');
 
+    // Edit the same gateway again; and check that the values were saved
+    // correctly.
+    $this->drupalGet('admin/commerce/config/payment-gateways/manage/example');
+    $this->assertSession()->fieldValueEquals('configuration[sips_merchant_id]', 'giraffe');
+    $this->assertSession()->fieldValueEquals('configuration[sips_passphrase]', 'kitten');
+    $this->assertSession()->fieldValueEquals('configuration[sips_payment_method]', '');
+
+    // Save the optional payment method field and check that has been saved
+    // correctly and that the other fields haven't been emptied.
+    $this->submitForm(['configuration[sips_payment_method]' => 'bunny'], 'Save');
+    $this->drupalGet('admin/commerce/config/payment-gateways/manage/example');
+    $this->assertSession()->fieldValueEquals('configuration[sips_merchant_id]', 'giraffe');
+    $this->assertSession()->fieldValueEquals('configuration[sips_passphrase]', 'kitten');
+    $this->assertSession()->fieldValueEquals('configuration[sips_payment_method]', 'bunny');
+
+    // Load the payment gateway through the API.
     $payment_gateway = PaymentGateway::load('example');
     $this->assertEquals('example', $payment_gateway->id());
     $this->assertEquals('SIPS', $payment_gateway->label());
     $this->assertEquals('sips_payment', $payment_gateway->getPluginId());
     $this->assertTrue($payment_gateway->status());
+
+    // Load the plugin from the gateway and check it's configuration.
     $payment_gateway_plugin = $payment_gateway->getPlugin();
     $this->assertEquals('0', $payment_gateway_plugin->getMode());
+    $this->assertEquals('SIPS Payment bunny', $payment_gateway_plugin->getDisplayLabel());
 
+    // Load the configuration of the gateway plugin and check that the values we
+    // set earlier are here.
     $configuration = $payment_gateway_plugin->getConfiguration();
     $this->assertEquals('llama', $configuration['sips_interface_version']);
     $this->assertEquals('kitten', $configuration['sips_passphrase']);
     $this->assertEquals('giraffe', $configuration['sips_merchant_id']);
     $this->assertEquals('owl', $configuration['sips_key_version']);
+    $this->assertEquals('bunny', $configuration['sips_payment_method']);
   }
 
 }