does anyone know how this works? I have an idea for a site, and I want to know if there are complicated things that I have to do on my end, or do I just redirect to the payment site. I am guessing that I'd have to add some data to the response.redirect string like payment total, user email, etc.
It depends on the gateway that you use. They are all setup a little differnt but most of the good ones ahve a set of data they require (they'll provide the names) covering all the basics like customer id, and/or invoice id, CC#, total, etc. Most also offer a template of the page that you can use as is or that you can customize to match your site's design(recommended).
It depends on the gateway more than the merchant account provider.