I don't get prices with Amazon Product Advertising API

Question

I try to get prices of an ASIN number with the Amazon Product Advertising API.

Code:

    $artNr = "B003TKSD8E";
    $base_url = "http://ecs.amazonaws.de/onca/xml";
    $params = array(
        'AWSAccessKeyId' => self::API_KEY,
        'AssociateTag' => self::API_ASSOCIATE_TAG,
        'Version' => "2010-11-01",
        'Operation' => "ItemLookup",
        'Service' => "AWSECommerceService",
        'Condition' => "All",
        'IdType' => 'ASIN',
        'ItemId' => $artNr);


    $params['Timestamp'] = gmdate("Y-m-d\TH:i:s.\\0\\0\\0\\Z", time());

    $url_parts = array();
    foreach(array_keys($params) as $key)
        $url_parts[] = $key . "=" . str_replace('%7E', '~', rawurlencode($params[$key]));
    sort($url_parts);

    $url_string = implode("&", $url_parts);
    $string_to_sign = "GET\necs.amazonaws.de\n/onca/xml\n" . $url_string;

    $signature = hash_hmac("sha256", $string_to_sign, self::API_SECRET, TRUE);

    $signature = urlencode(base64_encode($signature));

    $url = $base_url . '?' . $url_string . "&Signature=" . $signature;

    $response = file_get_contents($url);
    $parsed_xml = simplexml_load_string($response);

I think this should be correct - but I don't get offers in the response:

<?xml version="1.0" ?>
  <ItemLookupResponse xmlns="http://webservices.amazon.com/AWSECommerceService/2010-11-01">
    <OperationRequest>
      <RequestId>*********************</RequestId>
      <Arguments>
        <Argument Name="Condition" Value="All">
      </Argument>
        <Argument Name="Operation" Value="ItemLookup">
      </Argument>
        <Argument Name="Service" Value="AWSECommerceService">
      </Argument>
        <Argument Name="ItemId" Value="B003TKSD8E">
      </Argument>
        <Argument Name="IdType" Value="ASIN">
      </Argument>
        <Argument Name="AWSAccessKeyId" Value="*********************">
      </Argument>
        <Argument Name="Timestamp" Value="2011-11-29T01:49:01.000Z">
      </Argument>
        <Argument Name="Signature" Value="*********************">
      </Argument>
        <Argument Name="AssociateTag" Value="*********************">
      </Argument>
        <Argument Name="Version" Value="2010-11-01">
      </Argument>
    </Arguments>
      <RequestProcessingTime>0.0083130000000000</RequestProcessingTime>
    </OperationRequest>
    <Items>
      <Request>
        <IsValid>True</IsValid>
        <ItemLookupRequest>
          <Condition>All</Condition>
          <IdType>ASIN</IdType>
          <ItemId>B003TKSD8E</ItemId>
          <ResponseGroup>Small</ResponseGroup>
          <VariationPage>All</VariationPage>
        </ItemLookupRequest>
      </Request>
      <Item>
        <ASIN>B003TKSD8E</ASIN>
        <DetailPageURL>http://www.amazon.de/Apple-iPhone-4-32GB-schwarz/dp/B003TKSD8E%3FSubscriptionId%3DAKIAI6NFQHK2DQIPRUEQ%26tag%3Dbanholzerme-20%26linkCode%3Dxm2%26camp%3D2025%26creative%3D165953%26creativeASIN%3DB003TKSD8E</DetailPageURL>
        <ItemLinks>
          <ItemLink>
            <Description>Add To Wishlist</Description>
            <URL>http://www.amazon.de/gp/registry/wishlist/add-item.html%3Fasin.0%3DB003TKSD8E%26SubscriptionId%3DAKIAI6NFQHK2DQIPRUEQ%26tag%3Dbanholzerme-20%26linkCode%3Dxm2%26camp%3D2025%26creative%3D12738%26creativeASIN%3DB003TKSD8E</URL>
          </ItemLink>
          <ItemLink>
            <Description>Tell A Friend</Description>
            <URL>http://www.amazon.de/gp/pdp/taf/B003TKSD8E%3FSubscriptionId%3DAKIAI6NFQHK2DQIPRUEQ%26tag%3Dbanholzerme-20%26linkCode%3Dxm2%26camp%3D2025%26creative%3D12738%26creativeASIN%3DB003TKSD8E</URL>
          </ItemLink>
          <ItemLink>
            <Description>All Customer Reviews</Description>
            <URL>http://www.amazon.de/review/product/B003TKSD8E%3FSubscriptionId%3DAKIAI6NFQHK2DQIPRUEQ%26tag%3Dbanholzerme-20%26linkCode%3Dxm2%26camp%3D2025%26creative%3D12738%26creativeASIN%3DB003TKSD8E</URL>
          </ItemLink>
          <ItemLink>
            <Description>All Offers</Description>
            <URL>http://www.amazon.de/gp/offer-listing/B003TKSD8E%3FSubscriptionId%3DAKIAI6NFQHK2DQIPRUEQ%26tag%3Dbanholzerme-20%26linkCode%3Dxm2%26camp%3D2025%26creative%3D12738%26creativeASIN%3DB003TKSD8E</URL>
          </ItemLink>
        </ItemLinks>
        <ItemAttributes>
          <Manufacturer>Apple Computer</Manufacturer>
          <ProductGroup>CE</ProductGroup>
          <Title>Apple iPhone 4 32GB schwarz</Title>/ItemAttributes>
        </Item>
      </Items>
    </ItemLookupResponse>

Can someone please explain me why I don't get any price-information?

Thank you very much

Answer 1

To get prices for an item you need to include a ResponseGroup parameter of Offers or OfferFull and a MerchantId parameter of All:

$params = array(
    'AWSAccessKeyId' => self::API_KEY,
    'AssociateTag' => self::API_ASSOCIATE_TAG,
    'Version' => "2010-11-01",
    'Operation' => "ItemLookup",
    'Service' => "AWSECommerceService",
    'Condition' => "All",
    'IdType' => 'ASIN',
    'ResponseGroup' => 'Offers',                <- important
    'MerchantId' => 'All',                      <- important
    'ItemId' => $artNr);

If you don't include the MerchantId of All you will only get offer listings for Amazon (the default MerchantId). This will return the offer listings and your response will include something like the following:

<Items>
    ...
    <Item>
        ...
        <Offers>
            ...
            <Offer>
                ...
                <OfferListing>
                    <Price>
                        <Amount>1350</Amount>
                        <CurrencyCode>USD</CurrencyCode>
                        <FormattedPrice>$13.50</FormattedPrice>
                    </Price>
                    ...
                </OfferListing>
                ...
            </Offer>
       ...

See the documentation for the Offers Response Group for more information. Also see the Response Group request parameter description for the ItemLookup operation for more valid Response Groups.