Newbie Tracking Question

?=overture

[is300]

Ok, so i want to track my sales generated from overture and google Pay per click campaigns. I plan to design the link in my overture campaign to go to my main domain followed by?=overture.

My question is, once they go to another page in my site and buy something, that code goes away. Thus, how do I track which campaigns are generating my sales, and how do I keep these seperate?

I imagine there is a simple solution to this?

[roitracker]

4 possible solutions:
1. Use your log files & track sales back to the entry page.
2. Use session cookies & log files to track the source.
3. Set a cookie that captures the query string & retrieve the cookie at checkout.
4. Use PPC tracking software.