Interestingly enough, my wife booked a ticket to SEA yesterday for Wed through her work travel agency since it was a bus trip. She is 2P. She got seats assigned automatically on all 4 segments. 1 is on a 50 seat express, but the others were all booked into E+ automatically. 2 of the flights only had e+ available. Don't know if that had anything to do with it, or if the fact it was an E fare, or If maybe sw sort of travel agency thing, but there was definitely no extra charge. Didn't check to see if she could select a different E+ seat without charge.