My client has a license for the eCommerce app. Currently it doesn't support a completely free checkout option. You can give out promo codes, but it still requires at least a $1 minimum purchase to process.
We do currently utilize eCommerce along with Digital Download app for people to purchase his past archived radio shows.
What we need to be able to do is allow new registered users to choose any 2 shows they want to download for free. After getting two shows for free, they have to purchase any future shows. We will hire someone to build us the add-on or eCommerce extension so that we can offer a limited number shows of user-picked shows for free.

Please contact me if you are interested and have any further questions. Let me know your estimated cost for helping us program this to work.

Thank you,
Kari Skinner