I don't use the C5 add-on for digital fulfillment.
I found a better solution that is less expensive and completely automated. Take a look at Digivendor, which I've used with C5 sites, and have gotten great results. It doesn't integrate with C5, but it doesn't have to.
When a customer clicks a buy now cart button, the payment is processed, then the system automatically makes their download available in a protected download area, creates a user name and password, and sends the customer the address of the download address along with their user name and password.
Kate