Landing Page Cookie Code to track Sales

I have a website that sells a single product through the Yahoo payment gateway. I would like to be able to track separately the sales of 4 affiliates who each have there own landing page hosted on my site - each has a site extension of my URL. I'd like to be able to simply add a tracking code to each landing page that installs site wide cookies which can be set to last for a given time period that follow through to checkout. There is only 1 product sold, however at checkout any number of them can be purchased, so I'll also need the number sold or the amount of the sale. My affiliates would like to avoid using an ugly affiliate URL so just wondering if this is possible so they can simply send their traffic to the existing URL of their landing page and when their customer arrives the cookies are installed. Also, there will obviously have to be some interface view the sales stats.

I'm not much into code so just wondering what the easiest best of doing this is, any suggestions are much appreciated.