We're not really familiar with MacHeist, but at a glance right now, apparently they sell bundles of software that add more titles as more people buy into them?
If so, you'd need a way to bundle downloads, adding more items to the bundle as more units are sold, and you'd need a way to provide buyers who bought-in early with the additional items not included at the time they purchased, correct?
I think you would use some combination of our "Package product" and Updates features, perhaps using "Send free download link" to handle special cases individually:
I.e., you would upload each item as a separate product, then create a product for the bundle using the "Package files from other products" setting. As new achievements are unlocked, you'd modify the bundle product to include additional products' files and use Updates to send previous buyers a free link for the newly-added product.