Ecommerce add-on?

I'm looking for a 'shopping cart' type add on to allow a user to select and checkout items from a selection.  From what I've seen in the directory and online research, it doesn't appear there is such an add-on right now.  Hopefully I'm wrong?  

This seems to be a very common use case and I'm quite surprised this hasn't been done yet.

Hi,

I haven't seen one either; maybe it's because a shopping cart just doesn't generalize well? I'd be glad to be proven wrong, of course.

Best regards,
Olli