Hi Shane,
The easiest way to do a custom integration would be to use the custom redirect URL option in the settings, to send users through to a page that you control (your custom scripts). This way, once a user is done with their design, instead of being redirected to shopping cart, the user is redirected to any page you want, and then you can essentially take it from there.
The redirect itself will also contain information as GET or POST data, such as the design ID the user created, and other useful information about the design like the image URL, etc. From there you could proceed in your own environment however you like!
Good luck!