Pro-tip: You can get money back on flex fares if your trip becomes cheaper

I've done this twice now on the Northeast Regional day of, which seems to drop prices in an effort to get people to book up remaining seats. If you purchased a flex fare, modify your trip and if the ticket you purchased is now lower priced you can "change" to the same ticket and get a refund! Got $70 back for a 2 person trip earlier this week doing this.

Also it's nearly always worth it to pay for the flex fare in my experience. Took me too long to figure this out.