While I'm sure it's not up to the fulfillment tasks (yet), there's Handel. I'm trying to keep it small/simple and non-web dependent. I'm currently working on the checkout process as a plugin based pipeline, so ploppingin things like authorization/address validation/delivery/notification type things can just be dropped in.
It's still young, and admittedly it was derived from a need to convert and ASP site to Apache/ModPerl. I'd be interested in hearing what you find our there based on your requirements as that would be good into to have during development.
Along the lines of Interchange, there is also WebGUI which is highly extensable and may have a shopping at this point.
Update: From the WebGUI changlogs...
- Added a shopping cart system and salable products management system. This
completes the foundation of WebGUI's commerce system and will allow us to
add very rich commerce features in the 7.x series.
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