I uploaded a game that was shareware, but had to put it behind a paywall/download due to low sales.
One of my customers had, in fact, paid for the original embedded version, but now that link no longer works for them? The restricted setting says that owners should be able to access it, but he cannot.
I looked into making a download key for the downloadable version to give to him, but itch.io does not appear to support private messaging.
How can I help this previous customer get the downloadable version of the product they already purchased?

Links: Embedded(deprecated), Downloadable.

It appears you created an entirely different product page, which is why they would not have access. A purchase is tied to a specific product page.

Regarding contacting them, you can use the built in email tool to send an email to all buyers/donators/etc. I think the easiest way would be to send out an email saying something along the lines of: “Thank you for paying for XYZ, I’m switching over to paid, if you want a key for the new page please reply to me here <your email>”

You may also be able to access the buyer’s email address directly from the purchase details dialog on your dashboard. If it’s only one person you could email them directly.

Hope that helps!