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.