You can use the $25 certificate so you'll definitely need to call for that (so it ends up as $20 after the extra booking fee.

Can't help on whether it's a good mileage run. You may want to post it in that forum so you can get the most expert advice. (It sounds like a really good fare to me as you're getting PS one way). I do think you should book it right away. You never know when one of those segments might run out of one of the fare buckets you're pricing at.