Clickbank - Track buyers from IPN when vendor does not provide email address

I searched both Google and this forum for the answer to this before posting so if the answer is already here, I apologize and please point me in the correct direction.

I get traffic to my website by way of Google Adwords. I direct all of my traffic to a single landing page with an email signup incentive. After my potential customer signs up for my Aweber list I keep records of what Google ad they clicked to find me by using a custom field.

I have Clickbank affiliate links in my emails. The problem is that when I make a sale and I receive the IPN in my inbox, it doesn't include the email address in the payment information.

I would really like to get this information so that I can track which ads lead to customers that end up buying, and cross reference the time of sale against email opens so I know which emails are selling the most/least.

Since the vendor does not provide the email, this doesn't seem possible. But is there a way that I can pass a parameter to the vendor sale page that somehow ends up in my IPN email?