Looks like the discount cannot be applied to flights if the base fare total (before taxes are added to fares) is less than discount amount. The Saver fares would look to reduce it below that amount. Also, you can't combine a Saver fare in one direction with a Main or higher in the other due to fare rule combination restrictions. You can combine a Main fare in one direction with a Main or higher class in the other direction, but not with a Saver fare.