Create a script for customers to customize my Cafepress designs

Hello, I run a small Cafepress store. I want to integrate a function seamlessly into my site where people can add a name to the bottom part of my designs and the design is then loaded back into Cafepress where people can then check out and purchase their customized designs. There is a site on there right now that does EXACTLY what I am talking about. It is: [login to view URL] please look at it to see what I am talking about and notice how it goes seamlessly off of cafepress to another location to customize the design then back into cafepress to check out. I want this EXACT thing but integrated perfectly with my preexisting designed Cafepress store: [login to view URL] This needs to work with all future designs I upload as well and be easy to use. Let me know if you have more questions about this I would be happy to answer them.