After the buyer finishes checkout, and once the payment processor notifies us they have completed the buyer's payment, we grant the buyer access to a thank-you page we generate for their transaction, where we automatically present their unique download link(s).
Upon completion of payment, we also send the buyer a thank-you email message with a link to reach their thank-you/download page in case they did not proceed there after checkout, or in case they could not proceed because the payment processor was still working on their payment for a while after the buyer finished checkout.