Zen cart is certainly an option. To make zen Cart look like your site (the site you posted) will take some programing.
Alternatively, you can purchase a template that is similar (in layout) and just change the graphics /colors.
Feel free to PM me if you have any questions.
Using Zen cart and matching your exisiting design would entail changing the Zen cart header and footer and making some changes to the CSS.... nothing too terrible... for the graphics... To include a "custom" message or something like that on the cards themselves would require a short 2nd step.
Give us a shout if you feel it`s beyond your scope....