Product Advertising API returns errors in three categories so that you can easily determine how best to handle the problem:
2XX errors are caused by mistakes in the request. For example, your request might be missing a required parameter. The error message in the response gives a clear indication of what is wrong.
4XX errors indicate an issue with the request. A 403 indicates the request was not authenticated correctly.
5XX errors are transient errors reflecting an error internal to Amazon. A 503 error means that you are submitting requests too quickly and your requests are being throttled. If this is the case, you need to reduce the number of requests you are sending per second.