I would install wordpress for your site and then use a good store mod like i have ...
wp estore
http://www.tipsandtricks-hq.com/word...-securely-1059
very cheap and can use on multiple sites. i hate sounding like infomercial but the support is far superior. If i found an issue i emailed him and fix without either hrs or 1 day. Very good guy there. I use it for multiple downloads. Then you can style it to match your site ...
here is full working one on my site
I have designed more and more sites with wordpress front end and WP shop and so easy for people to use it ..also i do multiple downloads with my videos .
www.cg-hq.com