I'm actually not parsing the HTML at all. Not really. I'm using PhantomJS, which is a headless web browser. Each time I load a page, I inject jQuery and just do a simple JS command like
$('#whatevertheIDisfortheform').submit();
It's the equivalent of opening up your console on each page and interacting with it via jQuery rather than a mouse and keyboard.
So something has parsed the HTML - but at least you didn't have to reinvent that wheel :)
Still, you might want to look at https://affiliate-program.amazon.co.uk/gp/advertising/api/de... - the Product Advertising API looks like the latest evolution of the E-Commerce Service API and might let you do things like stop the bot from making duplicate similar purchases, or accidentally fulfilling your wishlist.
$('#whatevertheIDisfortheform').submit();
It's the equivalent of opening up your console on each page and interacting with it via jQuery rather than a mouse and keyboard.