I'd like to provide the amazon filler function on my site. How do I program it? If it is amazon API, which part? thanks.
If your talking about filleritem.com then its a website built on the API itself. You would have to code it into your site itself - take the amount you need to fill up to and return products for that value.
It is quite in depth to go into full code detail here. You first need to get your application talking to the Amazon API. Once you have done that, you will need to find a way to query the Amazon API for results matching a price (and other criteria, such as category). If you can't do that directly to Amazon, then you will have to do something like filtering results you return by the criteria. Think carefully about caching, number of calls to Amazon etc. as it can end up quite server intensive.