Congratulations on launching - I've been in a similar position with getting a HN spike and hitting email caps, but that's why I love Heroku (and SendGrid) for easily scaling things up, especially when I didn't really know what I was doing.

Do you guys have anything in place to alert you if you get another spike in traffic? If not, I'd recommend using Chartbeat and setting up alerts so that it emails/notifies your phone when you get a flood of traffic - you might find that press mentions pop up here and there and knowing about them straight away can help you fan the flames a bit. It's also pretty useful for when your site goes down as it instantly alerts you. Well done again - great job with launching.