Two years ago I was looking for something very similar myself and not finding it, decided I'd try to learn a bit of Python and try out Google App Engine simultaneously. This was the result:

http://www.ijustshipped.com

I am overhauling it at the moment because it needs a responsive design, SSL, and non-Google-reliant accounts (based on feedback I got when I initially released it).

I'd love to hear if anyone thinks they'd use this..