Need help setting up private page for sales download?

I’m a bit confused here.

Im using my wordpress blog to promote a new product…

For a new product that is in pdf form I am accepting payment via paypal for which i custom created an ‘add to cart’ button.

In the options there is a place where I can enter in the url that the customer will get forwarded too once payment is complete.

I would like to make this a thank you page of sorts. I would like it to send them to a simple page that asks them to punch in there first name and email in order to be immediately emailed there downloadable product.

I suppose the email could send them an email with all the pdf’s already attatched, or it could send them the link to the download page i could setup on my site along with the password.

My confusion comes in with how do i setup a private page on my site? i mean, i am running a wp blog, and if i create a post or page for the above mentioned thank you page that customers get forwarded to once the paypal is complete….what is stopping someone from finding that page randomly on my blog and punching in there email to get the product for free?

i know how to pass protect pages, but again, same issue as above, how would i create the private optin page so that others dont find it to enter there details w/o paying?

hope that wasnt too confusing as i figure this is really simple and im just overlooking it.

thanks