Actually, they sent me a letter in advance, asking for quarterly payments so there would not be a penalty. They even included payment vouchers and 4 envelopes!
It's my understanding that the penalty is waved the first year you hit the threshold, then you are responsible from that point on if they are owed.
I didn't even think about paying quarterly taxes until I came across this blog. I just started mystery shopping and completely forgot that it IS a legit stream of income. Also, seeing that my tax bracket changed, really opened my eyes. [www.libertytax.com]