I think you did good by adding the extra options.
"Powered by Ubercart, the free shopping cart software." is horrible but "Powered by UberCart" isnt that bad.
I would avoid the "free" word at all costs. "Open Source" has the same practical meaning and sounds much better, specially these days as open source software gains more and more awareness.
You see, after you setup a whole Drupal/Ubercart solution for your client, how can you charge him when it says "free" in the frontpage? Not everyone understands the work that goes behind these things.
As for giving back, i think its very important for us Ubercart users to be involved in the community.
For those who cant code, build some themes or contribute with translations.
Or donate some money.