I've just finished configuring my zen-cart (ver 1.2.2) for downloads. I read the FAQ on it, and read a lengthy post about it in this forum. Both helped, but I still got stuck. Here's how I fixed it. Just want to pass this on and possibly save a few gray hairs for someone.
Everything set up fine per the FAQ. Then I tested it with an order. Got to the final screen and everything was processed, and there at the bottom of the page was my download link. But when I clicked it, I got "Internal Server Error". Sounds like an apache problem to me ......
I run my own server, so tinkering around with apache was no problem. I try to run a fairly secure and restricted server, and it was one of these settings I must of made earlier that was hanging me up. The web home for zen-cart resided in /home/mydomain/www, and I had Apache set up to ignore OPTIONS for anything in /home/*/www. Well, zen-cart needed those OPTIONS.
So in httpd.conf, I added the following to the <VirtualHost> section ...
Restart apache and presto, it all worked fine.