diff --git a/tests/php/StoreApi/Routes/CartExtensions.php b/tests/php/StoreApi/Routes/CartExtensions.php
index 6f97ca842d8..3275073b06d 100644
--- a/tests/php/StoreApi/Routes/CartExtensions.php
+++ b/tests/php/StoreApi/Routes/CartExtensions.php
@@ -6,16 +6,50 @@
 namespace Automattic\WooCommerce\Blocks\Tests\StoreApi\Routes;
 
 use Automattic\WooCommerce\Blocks\Tests\StoreApi\Routes\ControllerTestCase;
+use Automattic\WooCommerce\Blocks\Tests\Helpers\FixtureData;
 
 /**
  * Cart Controller Tests.
  */
 class CartExtensions extends ControllerTestCase {
 
+	/**
+	 * Setup test products data. Called before every test.
+	 */
+	public function setUp() {
+		parent::setUp();
+
+		$fixtures = new FixtureData();
+
+		$this->product = $fixtures->get_simple_product(
+			array(
+				'name'          => 'Test Product 1',
+				'regular_price' => 10,
+			)
+		);
+
+		wc_empty_cart();
+
+		wc()->cart->add_to_cart( $this->product->get_id(), 1 );
+
+		woocommerce_store_api_register_update_callback(
+			array(
+				'namespace' => 'valid-test-plugin',
+				'callback'  => function() {
+					add_action(
+						'woocommerce_cart_calculate_fees',
+						function() {
+							wc()->cart->add_fee( 'Surcharge', 10, true, 'standard' );
+						}
+					);
+				},
+			)
+		);
+	}
 	/**
 	 * Test getting cart with invalid namespace.
 	 */
-	public function test_post() {
+	public function test_invalid_namespace() {
 		$request = new \WP_REST_Request( 'POST', '/wc/store/v1/cart/extensions' );
 		$request->set_header( 'Nonce', wp_create_nonce( 'wc_store_api' ) );
 		$request->set_body_params(
@@ -28,4 +62,26 @@ public function test_post() {
 			400
 		);
 	}
+
+	/**
+	 * Test getting cart with invalid namespace.
+	 */
+	public function test_cart_being_updated() {
+		$request = new \WP_REST_Request( 'POST', '/wc/store/v1/cart/extensions' );
+		$request->set_header( 'Nonce', wp_create_nonce( 'wc_store_api' ) );
+		$request->set_body_params(
+			array(
+				'namespace' => 'valid-test-plugin',
+			)
+		);
+		$this->assertAPIResponse(
+			$request,
+			200,
+			array(
+				'totals' => array(
+					'total_fees' => '1000',
+				),
+			)
+		);
+	}
 }