I am hoping to pass a username. I have an order history tab set up on my site and I try to match purchases to users with IP. But this is no where near a perfect method since some have settings to change IPs at https: sites, etc.
But now that I actually think about it, ejunkie would have to pick up the username when they click add to cart, only to send it to the payment processor, to send it back to ejunkie, to POST it to me. I think Ill be using IPs for awhile.