Hi,
I'm Alex, PHP developer.
So I imagine you will give unique ID for each website you'll give button to. So basically on the backend you will be able to see visitors from websites and see if they successfully purchased something - if yes, you will see that clearly. I can make it by colors: let's say if visitor purchased, it becomes green so that in the end of a month you see how much money each website earned you. How does it sound?
Thanks and let me know.
Alex