Sorry if I'm asking something obvious as I don't have a PayPal acct myself to root around in and they're online "Help Center" is not helping me.
My question is, does PayPal not have some type of mechanism to return a user to the seller's website with some kind of flag in the url to certify a successful transaction to thus allow the customer to download the product?
You can use Auto Return to automatically return the buyer to a URL after a successfuly payment. You can also use Payment Data Transfer (PDT) to transmit the details of the payment via the browser; however, this is not as reliable as the Instand Payment Notification (IPN) method of verifying successful payments.
You can find more information on both PDT and IPN on PayPal's Integration Center, and can test them in the PayPal Sandbox.