Well personally, the selling points for Ubercart for me are simply that it is free, open-source, easy to install with Drupal, and does the job. I think those are the things you should be promoting rather than kind of meaningless marketing speak about connectivity, etc. Those are the sorts of tag lines I completely tune out and I suspect I'm not the only one.

Keep it simple: Ubercart: The leading, open-source, drupal store module.

Or über, as you know, means "over" or "super" in German, so maybe play on that a bit.

Something like:
Ubercart,
Over-easy, under-priced.

Clever logo by the way Ãœ