Simply add a parameter to your URL (e.g. affid= germany email list 1234); write a little global include file that looks for the parameter in the URL, pulls it out, and stuffs it in a cookie. Then, use good old rel=canonical to tell the search engines that the canonical version of this page is the version without that affiliate ID parameter. Here's an example written in plain old ancient ASP: P.S. while calling the parameter "affid" probably makes this example more readable, if it were ME, I'd name the parameter something that looks less like an affiliate program IDTracking Sales Add a column for the affiliate ID to the database table where you track purchases.
At purchase time, suck the affiliate ID out of the cookie. (And, when you write the cookie initially, I recommend a 60 day or 90 day lifespan on the cookie so that your affiliates will get credit if the customer returns later and makes a purchase...affiliates like this ) I a ton of little affiliates that rarely generate any sales, plus a handful of affiliates that deliver 80% of the customers.