I'm following...
To be clear, this is the post or page that would be your initial 'sell' page; the page/post with the digital product listed
Or
would this be the page (ie the thank you page) that the customer is redirected to after the transaction?
and finally, wherever this code is, the WP Paypal plugin will find it and include it in the email sent to the customer?
Sorry, I'm a newbie of sorts with advanced WP functions. Thanks for your patience with these questions. This is the first obstacle I've come across in using WP.