I just thought I'd post that I received my I-797 receipt / extension letter today. So if anyone else received their biometrics first instead of the extension letter, it can happen.
It looks like they actually sent it out a day before the biometrics I-797c (which was dated the 10th), which is very strange ( the date on the envelope was the 9th - and I received it today, the 17th). I'm not sure why it took so long, but the main thing is that it got here!
I also noticed that the day they sent it out is the same day my check cleared from my bank, so I will use that as an indication for when they will send things out from now on.