diff --git a/app/Services/Stripe/FakeStripeClient.php b/app/Services/Stripe/FakeStripeClient.php
index c6bb114..34b9f81 100644
--- a/app/Services/Stripe/FakeStripeClient.php
+++ b/app/Services/Stripe/FakeStripeClient.php
@@ -161,7 +161,12 @@ class FakeStripeClient implements StripeClient
     public function createProduct(string $name, array $metadata = [], ?string $idempotencyKey = null): string
     {
         $key = IdempotencyKey::forProduct($idempotencyKey, $name, $metadata);
-        $parameters = ['name' => $name, 'metadata' => $metadata];
+
+        // The same parameters forProduct() just fingerprinted, asked for
+        // through the one method that knows what they are — a local literal
+        // here could drift from what the key actually covers without a test
+        // ever seeing it.
+        $parameters = IdempotencyKey::productParameters($name, $metadata);
 
         // Replays the first answer for a repeated key, as Stripe does.
         $replayed = $this->replay($key, $parameters);
@@ -187,13 +192,11 @@ class FakeStripeClient implements StripeClient
         ?string $idempotencyKey = null,
     ): string {
         $key = IdempotencyKey::forPrice($idempotencyKey, $productId, $amountCents, $currency, $interval, $metadata);
-        $parameters = [
-            'product' => $productId,
-            'unit_amount' => $amountCents,
-            'currency' => strtolower($currency),
-            'interval' => $interval,
-            'metadata' => $metadata,
-        ];
+
+        // Same reasoning as createProduct(): the parameters forPrice() just
+        // fingerprinted, not a second literal that has to be kept in step by
+        // hand.
+        $parameters = IdempotencyKey::priceParameters($productId, $amountCents, $currency, $interval, $metadata);
 
         $replayed = $this->replay($key, $parameters);
 
diff --git a/app/Services/Stripe/IdempotencyKey.php b/app/Services/Stripe/IdempotencyKey.php
index f42b79d..d71f8ea 100644
--- a/app/Services/Stripe/IdempotencyKey.php
+++ b/app/Services/Stripe/IdempotencyKey.php
@@ -38,19 +38,44 @@ final class IdempotencyKey
         string $interval,
         array $metadata,
     ): ?string {
-        return self::with($key, [
-            'product' => $productId,
-            'unit_amount' => $amountCents,
-            'currency' => strtolower($currency),
-            'interval' => $interval,
-            'metadata' => $metadata,
-        ]);
+        return self::with($key, self::priceParameters($productId, $amountCents, $currency, $interval, $metadata));
     }
 
     /** The key for a Product, for the same reason and against the same trap. */
     public static function forProduct(?string $key, string $name, array $metadata): ?string
     {
-        return self::with($key, ['name' => $name, 'metadata' => $metadata]);
+        return self::with($key, self::productParameters($name, $metadata));
+    }
+
+    /**
+     * What counts as "the same call" for a Price — public because
+     * FakeStripeClient has to ask this exact question too, to decide whether a
+     * repeated key is a retry or a changed call. Built inline a second time in
+     * the fake, it would only ever match this by coincidence, and a later
+     * change to what a Price call fingerprints could drift from what the fake
+     * detects without a single test noticing — which is exactly the gap this
+     * task exists to close.
+     */
+    public static function priceParameters(
+        string $productId,
+        int $amountCents,
+        string $currency,
+        string $interval,
+        array $metadata,
+    ): array {
+        return [
+            'product' => $productId,
+            'unit_amount' => $amountCents,
+            'currency' => strtolower($currency),
+            'interval' => $interval,
+            'metadata' => $metadata,
+        ];
+    }
+
+    /** What counts as "the same call" for a Product — same reason as priceParameters(). */
+    public static function productParameters(string $name, array $metadata): array
+    {
+        return ['name' => $name, 'metadata' => $metadata];
     }
 
     /**