I cant seem to find any fare that will work for the cert. I've tried it on both S-fares and on W-fares now. Does the coupon only work for fares above a certain amount? $200, etc? Or is it only available on the really high booking classes? I can't find any restrictions in the T&C , yet it doesnt work on any flights.

The little green star always seems to come up, indicating I can use the cert. But when I try to book, it says its not good for this itinerary.