Index: amazon_store.module
===================================================================
RCS file: /cvs/drupal-contrib/contributions/modules/amazon_store/amazon_store.module,v
retrieving revision 1.8.2.11
diff -u -r1.8.2.11 amazon_store.module
--- amazon_store.module	7 Apr 2010 21:00:42 -0000	1.8.2.11
+++ amazon_store.module	10 Oct 2010 10:53:53 -0000
@@ -704,7 +704,9 @@
   );
 
   $results = amazon_store_http_request('CartModify', $parameters);
-  $actual_quantity = $quantity > 0 ? (int)$results->Cart->CartItems->CartItem[0]->Quantity : $quantity;
+  foreach ($results->Cart->CartItems->CartItem as $cartitem) {
+    if ($cartitem->CartItemId == $cartItemId) $actual_quantity = $quantity > 0 ? (int)$cartitem->Quantity : $quantity;
+  }
 
   if (empty($results->error)) {
     drupal_set_message(t("Cart quantity updated to @quantity", array('@quantity' => $actual_quantity)));