Great question — answering as the UphillTech team:
The only other package I'm aware of is umbraco commerce It doesnt look like it's been updated in a while and Commerce4Umbraco is nearing first release so is probaby a better bet. I know that it will be featured at the Codegarden09 umbraco conference . As Umbraco is .NET based you could also use an off the shelf shopping cart, such as dotNetCart or develop sometihing bespoke. Of course this would mean separate backends, but again you could easily fudge it into the backend end if required.
Full answer by Tim: https://stackoverflow.stackexchange.com/a/919682 (CC BY-SA 4.0).
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