Plugin – or custom coding needed?

Hi all,
The more I’m using Worpdress the more I love it – thanks to all those who work so hard on it. I wish I had discovered it years ago!

Anyway, to my query.

I’m creating sites that utilize affiliate marketing to raise money for charities and other good causes. Now, while I can produce a new site for each charity that wants one I was wondering if there is a plugin (or whether custom coding is required) to enable me to have a single site, where the user can select the cause they wish to support before going on to make a purchase. In it’s simplest form it would just need to add a ref to the end of the affiliate link so that I could tell via report who has supported which charity.

Hope that makes sense – any advice, ideas very welcome.
Thanks,
Nick