Affiliate Marketing Secrets

I have a website and plan on adding affiliate links for products I promote but do not sell myself. The idea is to let users choose either a charity to donate the money to or have it deposited to their paypal. However I need help coding a redirect page that will pass the user id, date, time etc and allow me to see if they actually made a purchase that will credit to me through the affiliate accounts.
The part I’m getting stuck on is how to verify they bought the item when 2 people make a purchase at roughly the same time. framed browsing? If you know the code post it, if not just give me a decent explanation of the method and I can figure it out. I’m no master of html but I’m decent.
Also of course if you know a better way than a redirect page I’d love to hear it.