You can either go through billpay, as Creditaddict suggests, or you can give them a call and try to pay by phone through a rep. There have been a few times that I wanted to PIF and the website wouldn't let me, but a rep was able to take my payment by phone with no problem.
I don't know why the system sometimes doesn't allow you to pay online, but does allow you to pay by phone. The reps are always just as confused about this as I am.