diff --git a/src/v4/Endpoint/Booking/BookingRequest.php b/src/v4/Endpoint/Booking/BookingRequest.php
index 9df8f77..a022d27 100644
--- a/src/v4/Endpoint/Booking/BookingRequest.php
+++ b/src/v4/Endpoint/Booking/BookingRequest.php
@@ -70,14 +70,31 @@ public static function single(
     /** @return array<string, mixed> */
     public function toArray(): array
     {
+        // Bring's Booking API expects customerNumber inside each
+        // consignment's product object — not at the request root. Posting
+        // it at the root produces a per-consignment BOOK-INPUT-019
+        // ("Customer number must be provided") even though the SDK
+        // constructor enforces a non-empty value, because Bring reads it
+        // from the product slot and finds nothing there.
+        //
+        // We model customerNumber once on the request (it's per-request in
+        // practice) and inject it into each consignment's product at
+        // serialization time, so the public SDK shape stays unchanged.
+        $customerNumber = $this->customerNumber;
+        $consignments = array_map(
+            static function (Consignment $c) use ($customerNumber): array {
+                $arr = $c->toArray();
+                $arr['product']['customerNumber'] = $customerNumber;
+
+                return $arr;
+            },
+            $this->consignments,
+        );
+
         return [
             'schemaVersion' => $this->schemaVersion,
-            'consignments' => array_map(
-                static fn (Consignment $c): array => $c->toArray(),
-                $this->consignments,
-            ),
+            'consignments' => $consignments,
             'testIndicator' => $this->testIndicator,
-            'customerNumber' => $this->customerNumber,
         ];
     }
 }
diff --git a/tests/v4/Endpoint/Booking/BookingApiTest.php b/tests/v4/Endpoint/Booking/BookingApiTest.php
index 500c1ed..5c74c72 100644
--- a/tests/v4/Endpoint/Booking/BookingApiTest.php
+++ b/tests/v4/Endpoint/Booking/BookingApiTest.php
@@ -64,7 +64,8 @@ public function testBookSerialisesAndPostsToCorrectUrl(): void
 
         $body = json_decode((string) $req->getBody(), true);
         self::assertSame('1', $body['schemaVersion']);
-        self::assertSame('PARCELS_NORWAY-10001234567', $body['customerNumber']);
+        self::assertArrayNotHasKey('customerNumber', $body, 'customerNumber must not be at request root — Bring expects it under consignments[].product');
+        self::assertSame('PARCELS_NORWAY-10001234567', $body['consignments'][0]['product']['customerNumber']);
         self::assertTrue($body['testIndicator']);
         self::assertSame(Product::HOME_DELIVERY_PARCEL->value, $body['consignments'][0]['product']['id']);
         self::assertSame('EVARSLING', $body['consignments'][0]['product']['additionalServices'][0]['id']);