I've recently built an application that has a goal of helping Print-on-demand companies. The most important feature is that the application allows the clients of the POD to pay up-front for their orders and it acts as a single point of access, both for the company and for their clients.
The application must be integrated with the tools the POD company uses so it's not a one-size-fits-all solution. A test version of the application is currently live and can be accessed for trying it out/testing.
If you're interested about the application, feel free to send me a PM and we can talk more.