I recently did it, but I didn't wait for the Kiosk to prompt me. A few hours before flight when I checked in online I first went and upgraded a DFW-PHL leg. It was $30 x 3 stickers so after the upgrade confirmed I went and purchased the stickers.

I felt it a good value, though. I was coming off a longer itinerary and wanted a chance to rest and sleep a little easier. Plus, my initial ticket was paid by an employer anyway.