This page of the Options dialog lets you set the default settings for Amazon searches.
Default Amazon site and search index settings
The default site and search index choices that you make here will be used to preset the options for you when you create new Amazon searches. The Amazon search dialog lets you change the settings for each individual search though, so you can just select the settings that you plan to use most.
Each site has a different set of search indexes. As with the site, you can select the one you plan to use most.
The site setting that you make here will also be used to control the Amazon search in the Quick Search toolbar.