Checkout 'credit' from another site

[sewwhattom]

I have a couple of sites I want to use for my one store. If someone goes to xyzstore.c0m I want to forward it to xyz.c0m. That I can do. What I want is for that session if there is a purchase in the email sent to me tell me it came from someone who typed in the xyzstore.c0m and was forwarded. I was thinking in the forward page of xyzstore.c0m to define a string that carried over to the xyz.c0m and was put in the e mail during checkout. I just dont know where to start. And it doesn't need to be a stored, just included in the e mail that one time.

Thanks, Tom