I get that. Even before I have the user create an account or login though, I need to grab the product that the user wants to purchase. So I have a page currently populated with static Paypal buttons (http://www.theharmonyman.com/browse-music.php) which all add those products to the paypal cart function.
I imagine I would replace these buttons with my own buttons that somehow cache the product info (of some sort?) and take the user to a page to login/create account, and eventually add to paypal's cart.
Once the payment is done, I need to be returned to this site to set a bit in a Sales table for the user unique ID, and then send an email to them with links to the music they've purchased (PDF).