Add a validate handler that checks the value in the amount field. Reject form submission if zero.

authorizenetwebform.module

@@ -174,11 +174,43 @@ function authorizenetwebform_form_webform_client_form_alter(&$form, &$form_state
     'type' => 'setting'
   );
 
-  // Submit handler. Adding it here the *second* submit handler.
+  // Add validation handler.
+  $form['#validate'][] = 'authorizenetwebform_webform_client_form_validate';
+
+  // Add submit handler. Adding it here the *second* submit handler.
   $first = array_shift($form['#submit']);
   array_unshift($form['#submit'], $first, 'authorizenetwebform_webform_client_form_submit');
 }
 
+/**
+ * Webform validate handler.
+ *
+ * @see authorizenetwebform_form_webform_client_form_alter().
+ */
+function authorizenetwebform_webform_client_form_validate($form, &$form_state) {
+
+  // Get key mappings for this webform.
+  $nid = $form_state['values']['details']['nid']; // Nice shorthand.
+  $variable_name = 'authorizenetwebform_key_map_' . $nid;
+  // Get the mapping of webform field => AuthNet field key.
+  $field_map = variable_get($variable_name, array());
+  // Get the mapping of AuthNet field key => webform field.
+  $flipped_map = array_flip($field_map);
+
+  // Get the amount.
+  $field_values = _authorizenetwebform_flatten_form_values($form_state['values']['submitted']);
+  $amount_key = $flipped_map['x_amount'];
+  $amount = $field_values[$amount_key];
+
+  // Do not allow submission with zero dollar amount.
+  if (empty($ammount)) {
+    $string = t('The amount must be greater than 0.');
+    drupal_set_message($string, 'error');
+    // Stop the submission.
+    $form_state['rebuild'] = TRUE;
+  }
+}
+
 /**
  * Webform submit handler.
  *
@@ -400,9 +432,12 @@ function authorizenetwebform_webform_client_form_submit($form, &$form_state) {
 
   if ($response_array['messages']['resultCode'] == 'Ok') {
 
-    // Extrat the response info we need for our records.
-    $response_info = $response_array['transactionResponse'];
+    $text = '';
     $failure = FALSE;
+    // Extrat the response info we need for our records.
+    if (array_key_exists('transactionResponse', $response_array)) {
+      $response_info = $response_array['transactionResponse'];
+    }
 
     // Decliend || Error.
     if ($response_info['responseCode'] == 2 || $response_info['responseCode'] == 3) {
@@ -486,9 +521,9 @@ function authorizenetwebform_webform_client_form_submit($form, &$form_state) {
       // @todo: Save this data into a more permanent location than watchdog?
 
       // Also print response to screen.
-      if (module_exists('devel')) {
-        dpm($response_array);
-      }
+      //if (module_exists('devel')) {
+        //dpm($response_array);
+      //}
     }
   }