Hm.
This depends on how flexible you want it to be. If you just want to upload an image then add text, perhaps with templates, you can do something server-side (PHP, ASP, Perl, &c.). Otherwise (placing the text, editing the image, adding sound, video, whatever), Java. You can use object serialization to send the card to a server-side script, which could make an image of it, or convert it to an image first using image-editing techniques. I suggest the former, because otherwise someone could capture the image between the server and client applet and just print it, but the latter would be easier to implement.
It might even be possible with JavaScript, at a stretch. I think it would be tricky, though.
One thing you will definitely need, though, is a server-side script. Quite apart from communicating with your e-commerce software, you'll need it to store the image, and create the image if using JavaScript or the second Java method, both of which wouldn't actually create an image but a set of properties that the script would use to create the image.