The Product Advertising API operation, Help, returns all of the required and optional input parameters for Product Advertising API operations, and all of the item attributes returned by Product Advertising API response groups. For example, to see the parameters for CartAdd , use the following request.

http://ecs.amazonaws.com/onca/xml?
Service=AWSECommerceService&
AWSAccessKeyId=[AWS Access Key ID]&
Operation=Help&
HelpType=Operation&
About=CartAdd

A snippet from the response shows the operation's parameters.

<OperationInformation>
  <Name>CartAdd</Name>
  <RequiredParameters>
    <Parameter>AssociateTag</Parameter>
    <Parameter>CartId</Parameter>
    <Parameter>HMAC</Parameter>
    <Parameter>Items</Parameter>
  </RequiredParameters>
  <AvailableParameters>
    <Parameter>AWSAccessKeyId</Parameter>
    <Parameter>ContentType</Parameter>
    <Parameter>Marketplace</Parameter>
    <Parameter>MarketplaceDomain</Parameter>
    <Parameter>MergeCart</Parameter>
    <Parameter>Style</Parameter>
    <Parameter>Validate</Parameter>
    <Parameter>Version</Parameter>
    <Parameter>XMLEscaping</Parameter>
  </AvailableParameters>

HelpType is either "Operation" or "ResponseGroup." The About parameter is set to an Product Advertising API operation or response group.