diff --git a/README.md b/README.md
index baf6094..3e54be8 100644
--- a/README.md
+++ b/README.md
@@ -57,7 +57,7 @@ $result = $transaction
         'customer_name' => 'Nama Pelanggan',
         'customer_email' => 'email@konsumen.id',
         'customer_phone' => '081234567890',
-        'expired_time' => Helper::makeTimestamp('6 HOUR'),
+        'expired_time' => Helper::makeTimestamp('6 HOUR'), // see Supported Time Units
     ]);
 
 echo $result->getBody()->getContents();
@@ -70,3 +70,10 @@ echo json_encode($debugs, JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES);
 ```
 
 Please check the `/examples` for the other examples
+
+## Supported Time Units
+> :exclamation: All time units are in a singular noun
+- SECOND
+- MINUTE
+- HOUR
+- DAY
diff --git a/examples/callback-handling.php b/examples/callback-handling.php
index 4ba751e..8d63938 100644
--- a/examples/callback-handling.php
+++ b/examples/callback-handling.php
@@ -16,7 +16,7 @@
  *
  * !! WARNING !!
  * Only enable it while debugging.
- * Leaving it enabled can lead to security compromise
+ * Leaving it enabled can lead to security issue
  */
 $callback->enableDebug();
 
diff --git a/src/Callback.php b/src/Callback.php
index 8411e4b..93c5400 100644
--- a/src/Callback.php
+++ b/src/Callback.php
@@ -82,9 +82,9 @@ public function localSignature(): string
     /**
      * Get incoming signature
      *
-     * @return string|null
+     * @return string
      */
-    public function incomingSignature(): ?string
+    public function incomingSignature(): string
     {
         return (string) (isset($_SERVER['HTTP_X_CALLBACK_SIGNATURE']) ? $_SERVER['HTTP_X_CALLBACK_SIGNATURE'] : "");
     }
diff --git a/src/Client.php b/src/Client.php
index 45e08f7..0a332af 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -120,8 +120,8 @@ public function __construct(...$args)
 
         $this->merchantCode = (string) is_array($args[0]) ? $args[0]['merchant_code'] : $args[0];
         $this->apiKey = (string) is_array($args[0]) ? $args[0]['api_key'] : $args[1];
-        $this->privateKey = (string) is_array($args[0]) ? $args[0]['private_key'] : $args[1];
-        $this->mode = (string) is_array($args[0]) ? $args[0]['mode'] : $args[2];
+        $this->privateKey = (string) is_array($args[0]) ? $args[0]['private_key'] : $args[2];
+        $this->mode = (string) is_array($args[0]) ? $args[0]['mode'] : $args[3];
 
         $baseUri = ($this->mode == Constant::MODE_DEVELOPMENT)
             ? Constant::URL_DEVELOPMENT
@@ -150,7 +150,7 @@ public function __construct(...$args)
             },
             'headers' => [
                 'Authorization' => 'Bearer ' . $this->apiKey,
-                'User-Agent' => 'zerosdev/tripay-sdk-php',
+                'User-Agent' => 'github:zerosdev/tripay-sdk-php',
             ]
         ];
 
@@ -165,19 +165,40 @@ public function __construct(...$args)
         $this->client = $this->createHttpClient($options);
     }
 
+    /**
+     * Create HTTP client
+     *
+     * @param array $options
+     * @return HttpClient
+     */
     private function createHttpClient(array $options): HttpClient
     {
         return new HttpClient($options);
     }
 
-    public function get($endpoint, array $headers = []): Response
+    /**
+     * Performe GET request
+     *
+     * @param string $endpoint
+     * @param array $headers
+     * @return Response
+     */
+    public function get(string $endpoint, array $headers = []): Response
     {
         return $this->client->get($endpoint, [
             'headers' => $headers,
         ]);
     }
 
-    public function post($endpoint, array $payloads, array $headers = []): Response
+    /**
+     * Performe POST request
+     *
+     * @param string $endpoint
+     * @param array $payloads
+     * @param array $headers
+     * @return Response
+     */
+    public function post(string $endpoint, array $payloads = [], array $headers = []): Response
     {
         return $this->client->post($endpoint, [
             'json' => $payloads,
@@ -185,6 +206,11 @@ public function post($endpoint, array $payloads, array $headers = []): Response
         ]);
     }
 
+    /**
+     * Get debug data
+     *
+     * @return object
+     */
     public function debugs(): object
     {
         return (object) $this->debugs;
diff --git a/src/Support/Helper.php b/src/Support/Helper.php
index d6d28e9..a0aed69 100644
--- a/src/Support/Helper.php
+++ b/src/Support/Helper.php
@@ -19,8 +19,7 @@ public static function makeSignature(Client $client, array $payloads): string
     {
         $merchantRef = isset($payloads['merchant_ref']) ? $payloads['merchant_ref'] : null;
         $amount = self::formatAmount($payloads['amount']);
-
-        $payloads['amount'] = self::formatAmount($payloads['amount']);
+        
         return hash_hmac('sha256', $client->merchantCode . $merchantRef . $amount, $client->privateKey);
     }
 
@@ -73,7 +72,7 @@ public static function checkRequiredPayloads(array $requireds, array $payloads):
 
     /**
      * Make unix timestamp
-     * Supported unit: SECOND, MINUTE, HOUR, DAY
+     * Supported time unit: SECOND, MINUTE, HOUR, DAY
      * i.e: "1 DAY", "13 HOUR", etc
      *
      * @param string $value
@@ -98,7 +97,7 @@ public static function makeTimestamp(string $value): int
         $supportedUnits = ['SECOND', 'MINUTE', 'HOUR', 'DAY'];
 
         if (!in_array($unit, $supportedUnits)) {
-            throw new InvalidArgumentException('Unexpected unit. Supported: ' . implode(', ', $supportedUnits));
+            throw new InvalidArgumentException('Unsupported time unit. Supported: ' . implode(', ', $supportedUnits));
         }
 
         switch ($unit) {