diff --git a/services/nginx/app/classes/customer_order_product_policy.php b/services/nginx/app/classes/customer_order_product_policy.php
new file mode 100644
index 00000000..cca75373
--- /dev/null
+++ b/services/nginx/app/classes/customer_order_product_policy.php
@@ -0,0 +1,98 @@
+<?php
+
+namespace classes;
+
+use RuntimeException;
+
+class customer_order_product_policy
+{
+    public const ONLY_TANKCLEANING_ATTRIBUTE = 'onlyTankCleaning';
+    public const ONLY_TANKCLEANING_MESSAGE = 'Only tankcleaning customers can only have tankcleaning products in their orders.';
+
+    public static function assertOrderAllowsProduct(int $orderId, int $productId): void
+    {
+        $message = self::orderProductViolationMessage($orderId, $productId);
+        if ($message !== null) {
+            throw new RuntimeException($message);
+        }
+    }
+
+    public static function orderProductViolationMessage(int $orderId, int $productId): ?string
+    {
+        $context = self::loadOrderProductContext($orderId, $productId);
+        if ($context === null) {
+            return null;
+        }
+        if ((int)($context['product_id'] ?? 0) < 1) {
+            return null;
+        }
+
+        return self::onlyTankCleaningViolation((bool)((int)($context['has_only_tank_cleaning'] ?? 0)), $context)
+            ? self::ONLY_TANKCLEANING_MESSAGE
+            : null;
+    }
+
+    public static function onlyTankCleaningViolation(bool $customerHasOnlyTankCleaning, array $productRow): bool
+    {
+        return $customerHasOnlyTankCleaning && !self::isTankCleaningProductRow($productRow);
+    }
+
+    public static function isTankCleaningProductRow(array $row): bool
+    {
+        return (int)($row['product_category'] ?? $row['category'] ?? 0) === 5
+            || self::rowMatchesProductTerms($row, ['tank cleaning', 'tankcleaning', 'tankrens']);
+    }
+
+    private static function loadOrderProductContext(int $orderId, int $productId): ?array
+    {
+        global $db;
+
+        if ($orderId < 1 || $productId < 1) {
+            return null;
+        }
+
+        $sql = "
+            SELECT
+                o.id AS order_id,
+                o.customer_id AS customer_number,
+                p.id AS product_id,
+                p.name AS product_name,
+                p.category AS product_category,
+                c.name AS category_name,
+                MAX(CASE WHEN ca.attribute = '" . self::ONLY_TANKCLEANING_ATTRIBUTE . "' THEN 1 ELSE 0 END) AS has_only_tank_cleaning
+            FROM orders o
+            LEFT JOIN products p ON p.id = {$productId}
+            LEFT JOIN categories c ON c.id = p.category
+            LEFT JOIN users u ON u.customer_number = o.customer_id
+            LEFT JOIN customer_attributes ca ON ca.user_id = u.id
+                AND ca.attribute = '" . self::ONLY_TANKCLEANING_ATTRIBUTE . "'
+            WHERE o.id = {$orderId}
+            GROUP BY o.id, o.customer_id, p.id, p.name, p.category, c.name
+            LIMIT 1
+        ";
+
+        $result = $db->query($sql);
+        if (!$result || $result->num_rows < 1) {
+            return null;
+        }
+
+        $row = $result->fetch_assoc();
+        return is_array($row) ? $row : null;
+    }
+
+    private static function rowMatchesProductTerms(array $row, array $terms): bool
+    {
+        $haystack = strtolower(trim(
+            (string)($row['product_name'] ?? $row['name'] ?? '') . ' ' .
+            (string)($row['category_name'] ?? '')
+        ));
+
+        foreach ($terms as $term) {
+            if ($term !== '' && str_contains($haystack, strtolower($term))) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+}
diff --git a/services/nginx/app/classes/invoice_period_flag_service.php b/services/nginx/app/classes/invoice_period_flag_service.php
index bb8e0a56..09778baf 100644
--- a/services/nginx/app/classes/invoice_period_flag_service.php
+++ b/services/nginx/app/classes/invoice_period_flag_service.php
@@ -2036,8 +2036,7 @@ class invoice_period_flag_service
 
     private function rowIsTankCleaningProduct(array $row): bool
     {
-        return (int)($row['product_category'] ?? 0) === 5
-            || $this->rowMatchesProductTerms($row, ['tank cleaning', 'tankcleaning', 'tankrens']);
+        return customer_order_product_policy::isTankCleaningProductRow($row);
     }
 
     private function isIncludedOrderItem(array $row): bool
diff --git a/services/nginx/app/objects/order_items_o.php b/services/nginx/app/objects/order_items_o.php
index 38a43603..248a4603 100644
--- a/services/nginx/app/objects/order_items_o.php
+++ b/services/nginx/app/objects/order_items_o.php
@@ -3,6 +3,7 @@
 namespace objects;
 
 use classes\db;
+use classes\customer_order_product_policy;
 use classes\object_property;
 use Exception;
 use traits\db_object_t;
@@ -93,6 +94,7 @@ class order_items_o extends db
     {
         global $db, $response;
         try {
+            customer_order_product_policy::assertOrderAllowsProduct($order_id, $product_id);
             // Avoid SQL injection
             $reference = $db->escape_string($reference);
             $notes = $db->escape_string($notes);
@@ -167,6 +169,7 @@ class order_items_o extends db
         try {
             // Get the order
             $order = (new orders_o())->getOrderById($order_id);
+            customer_order_product_policy::assertOrderAllowsProduct($order_id, $product_id);
             // Get the product price
             $price = (new products_o())->getProductById($product_id)->getDepartmentPrice((int)$order->department_id->value());
 
@@ -354,4 +357,4 @@ class order_items_o extends db
     {
         return (new products_o())->select((int)$this->product_id->value());
     }
-}
\ No newline at end of file
+}
diff --git a/services/nginx/app/tests/Api/OrderItemsApiTest.php b/services/nginx/app/tests/Api/OrderItemsApiTest.php
index 577c80f6..5e21cacc 100644
--- a/services/nginx/app/tests/Api/OrderItemsApiTest.php
+++ b/services/nginx/app/tests/Api/OrderItemsApiTest.php
@@ -15,7 +15,6 @@ it('requires notes when adding the extraordinary chemistry product to an order',
         'reference' => 'NOTE-REQUIRED',
     ]);
     $product = api_fixtures()->createProduct([
-        'id' => 902701,
         'name' => \objects\products_o::EXTRAORDINARY_CHEMISTRY_PRODUCT_NAME,
         'price' => 299,
         'requires_note' => 0,
@@ -49,6 +48,85 @@ it('requires notes when adding the extraordinary chemistry product to an order',
     expect($response->data()['notes'] ?? null)->toBe('Graffiti removal on left side');
 });
 
+it('only allows tankcleaning products for only tankcleaning customers', function (): void {
+    api_test_covers('POST /order/items', 'customer_rules');
+
+    $customer = api_fixtures()->createUser(['display_name' => 'Only Tankcleaning Customer']);
+    api_fixtures()->addCustomerAttribute((int)$customer['id'], 'onlyTankCleaning');
+    $department = api_fixtures()->createDepartment();
+    $order = api_fixtures()->createOrder([
+        'customer_id' => $customer['customer_number'],
+        'department_id' => $department['id'],
+        'reference' => 'ONLY-TANK',
+    ]);
+    $washProduct = api_fixtures()->createProduct([
+        'name' => 'Forvogn',
+        'price' => 649,
+        'category' => 4,
+    ]);
+    $tankCleaningProduct = api_fixtures()->createProduct([
+        'name' => 'Saebe/kemi, 1-4 spulehoveder',
+        'price' => 299,
+        'category' => 5,
+    ]);
+    $session = api_fixtures()->createUserSession([], ['group_id' => 1]);
+
+    api_client()
+        ->post('/order/items', [
+            'order_id' => $order['id'],
+            'product_id' => $washProduct['id'],
+            'quantity' => 1,
+        ], $session['headers'])
+        ->assertStatus(400)
+        ->assertEnvelope()
+        ->assertSuccess(false)
+        ->assertMessage(\classes\customer_order_product_policy::ONLY_TANKCLEANING_MESSAGE);
+
+    $response = api_client()->post('/order/items', [
+        'order_id' => $order['id'],
+        'product_id' => $tankCleaningProduct['id'],
+        'quantity' => 1,
+    ], $session['headers']);
+
+    $response
+        ->assertStatus(200)
+        ->assertEnvelope()
+        ->assertSuccess();
+
+    expect((int)($response->data()['product_id'] ?? 0))->toBe((int)$tankCleaningProduct['id']);
+});
+
+it('allows non-tankcleaning products for customers without the only tankcleaning attribute', function (): void {
+    api_test_covers('POST /order/items', 'customer_rules');
+
+    $customer = api_fixtures()->createUser(['display_name' => 'Regular Order Item Customer']);
+    $department = api_fixtures()->createDepartment();
+    $order = api_fixtures()->createOrder([
+        'customer_id' => $customer['customer_number'],
+        'department_id' => $department['id'],
+        'reference' => 'REGULAR-WASH',
+    ]);
+    $washProduct = api_fixtures()->createProduct([
+        'name' => 'Forvogn',
+        'price' => 649,
+        'category' => 4,
+    ]);
+    $session = api_fixtures()->createUserSession([], ['group_id' => 1]);
+
+    $response = api_client()->post('/order/items', [
+        'order_id' => $order['id'],
+        'product_id' => $washProduct['id'],
+        'quantity' => 1,
+    ], $session['headers']);
+
+    $response
+        ->assertStatus(200)
+        ->assertEnvelope()
+        ->assertSuccess();
+
+    expect((int)($response->data()['product_id'] ?? 0))->toBe((int)$washProduct['id']);
+});
+
 it('does not allow clearing notes for order items whose product requires notes', function (): void {
     api_test_covers('PUT /order/items', 'validation');
 
@@ -62,7 +140,6 @@ it('does not allow clearing notes for order items whose product requires notes',
         'reference' => 'NOTE-EDIT',
     ]);
     $product = api_fixtures()->createProduct([
-        'id' => 902702,
         'name' => 'API Note Required Product',
         'price' => 199,
         'requires_note' => 1,
@@ -75,7 +152,7 @@ it('does not allow clearing notes for order items whose product requires notes',
         'quantity' => 1,
         'notes' => 'Initial note',
     ]);
-    $session = api_fixtures()->createUserSession(['edit_order_items']);
+    $session = api_fixtures()->createUserSession(['edit_order_items', 'list_order_items']);
 
     api_client()
         ->put('/order/items', [
@@ -95,7 +172,6 @@ it('returns the extraordinary chemistry product with requires_note enabled', fun
     api_test_covers('GET /products', 'happy');
 
     $product = api_fixtures()->createProduct([
-        'id' => 902703,
         'name' => \objects\products_o::EXTRAORDINARY_CHEMISTRY_PRODUCT_NAME,
         'price' => 299,
         'requires_note' => 0,
diff --git a/services/nginx/app/tests/Unit/Orders/CustomerOrderProductPolicyTest.php b/services/nginx/app/tests/Unit/Orders/CustomerOrderProductPolicyTest.php
new file mode 100644
index 00000000..8da99e2b
--- /dev/null
+++ b/services/nginx/app/tests/Unit/Orders/CustomerOrderProductPolicyTest.php
@@ -0,0 +1,40 @@
+<?php
+
+declare(strict_types=1);
+
+use classes\customer_order_product_policy;
+
+it('recognizes tankcleaning products by category and legacy names', function (): void {
+    expect(customer_order_product_policy::isTankCleaningProductRow([
+        'product_category' => 5,
+        'product_name' => 'Saebe/kemi, 1-4 spulehoveder',
+        'category_name' => 'Other',
+    ]))->toBeTrue()
+        ->and(customer_order_product_policy::isTankCleaningProductRow([
+            'product_category' => 3,
+            'product_name' => 'Tank cleaning 4 spulehoveder',
+            'category_name' => 'Other',
+        ]))->toBeTrue()
+        ->and(customer_order_product_policy::isTankCleaningProductRow([
+            'product_category' => 3,
+            'product_name' => 'Saebe/kemi, 1-4 spulehoveder',
+            'category_name' => 'Tankrens',
+        ]))->toBeTrue();
+});
+
+it('detects only tankcleaning violations only for attributed customers and non-tank products', function (): void {
+    $washProduct = [
+        'product_category' => 4,
+        'product_name' => 'Forvogn',
+        'category_name' => 'Udvendig',
+    ];
+    $tankCleaningProduct = [
+        'product_category' => 5,
+        'product_name' => 'Tank cleaning 4 spulehoveder',
+        'category_name' => 'Tank cleaning',
+    ];
+
+    expect(customer_order_product_policy::onlyTankCleaningViolation(true, $washProduct))->toBeTrue()
+        ->and(customer_order_product_policy::onlyTankCleaningViolation(true, $tankCleaningProduct))->toBeFalse()
+        ->and(customer_order_product_policy::onlyTankCleaningViolation(false, $washProduct))->toBeFalse();
+});