I want to search Amazon’s product API for discounted products (30% off or more). Is there a simple way to do this via the API?

For certain search indexes, you can ask for minimum percentage off:

http://docs.amazonwebservices.com/AWSECommerceService/2011-08-01/DG/USSearchIndexParamForItemsearch.html

For instance, if I’m searching for Toys, I can get the minimum percentage off with the following parameters:

{“SearchIndex”: “Toys”, “Service”: “AWSECommerceService”, “AWSAccessKeyId”: “xxx”, “Timestamp”: “2011-12-26T05:41:20Z”, “Version”: “2009-11-02”, “Keywords”: “Halo”, “AssociateTag”: “xxx”, “MinPercentageOff”: 30, “Operation”: “ItemSearch”, “ResponseGroup”: “Large”}

However, if I’m searching for video games, the “MinPercentageOff” parameter is ignored. The documentation page I included above shows which of the search indices accept this parameter – note that if the index doesn’t support it it’s simply ignored (the search doesn’t fail, it just doesn’t honor that parameter).