I don't thing wget can handle POST data, but my favorite app for this sort of thing, curl, can do just about anything that a browser can do. I haven't looked at amazon.com, but I can give you an outline on how to approach things:
First find out what data is sent to amazon by your browser. I usually use netcat for this.
- open the search site at amazon
- have netcat listen to some local port
- configure your browser to user 127.0.0.1:<port> as proxy
- enter search data and click send
Now the data should be in the console window where you run netcat. The first line will tell you weather it is a GET or a POST request, and the last line contains the search data: opt1=arg1&opt2=arg2...
Finally, test curl (use -d <data> to specify the search data)
If you are lucky this works. If not, you have to make curl do more advanced simulation of your browser. (e.g. handle cookies, set referrer address, ...)
For more information on using curl, look at: http://curl.haxx.se/docs/httpscripting.html