We have a case which would be perfectly solved if $0.00 sales didn't require a card. Its 24-hr link expiration is key. As a record label w/digital downloads we sell for real $$$ but we depend on reviewers who get it free. The direct file download allows link re-distribution, even if we put it behind a locked page, the content sub-link would still work, no?
We're looking at linking to box.com which will auto-expire links. But that's another system to maintain. I get that music is easily shared but it's one preventative step more to share actual hefty files than it is to forward text links.