Tried using the latest version (http://www.ubercart.org/contrib/355#comment-5313) in Drupal 5.5 with Ubercart 1.0RC4 with two options:
min max flat percent 0.00 99.99 5 0 100.00 1000000.00 0 0
However it would only ever pick the first option regardless of the order total.
Interestingly, after applying the patch in http://www.ubercart.org/contrib/355#comment-17550 it now works.
