'S' class tickets are no problem to use $off discount coupons.

The primary issue is availability.

I have used the CS25s several times for dropping Bay Area to LA area tix to near E-far prices; my suggestion is as follows:

1) Make your reservation via telephone - make sure you get the fare class and price you see via internet or whatever
2) Don't pay, but hold it for 24 hours
3) Go to the nearest United ticket counter and pay + use the discount

The reason for this is that while I've never had problem booking flights, I HAVE had difficulty with some agents not being able to find the fare I discovered on the internet - even WITH printouts of the flights in questions. Apparently there is some extra work needed with inexperienced agents might not know.

Good luck...