Could you upload to dropbox, generate a link and then use that link for download? Tip: Every dropbox link ends in a "0" and if you change that "0" to a "1" it initiates automatic download of that product anytime someone clicks the link.
I'm currently selling digital products (some up to 20GB) on Shopify, but it's still got it's woes for shop owners. I came here to see what the main issues would be if I switch over to SS. I'm curious if there's anyway to get that customer the dropbox link in an automated email after they purchase.