The closest we can suggest is using cross-promotion on the thank-you (download link) page itself. You could use the built-in cross-promotion feature we provide in our product configuration screens (where you configure each product's thank-you page to promote a selection of your other products), or you can just add custom HTML to your thank-you page to provide a link to an "OTO" page on your site, where the buyer could then Add to Cart or Buy Now your special-offer product. The special-offer product could be either a unique product not offered elsewhere, or an existing "regular" product with a secret Discount Code you only reveal on the thank-you page or email, or on your OTO page.
