[protomyth]

Actually, what you wrote there has some false parts. They send a certified letter and will put a lien on you bank account for the amount if you don't respond fast enough[1]. They add interest immediately, there is no grace period. You can arrange monthly payments (they are required to let you do this the first time). They will deduct the money you owe from the next year's return.

1) Fast enough can get a little weird. Had to loan a friend money because of a mistake and he didn't even get the letter before the lien happened. Saw him pull up his bank account on a Monday morning after his card was rejected at breakfast. He came in the next day with the certified letter and listened to him call the IRS about the problem with his return. They did get new computers a couple years ago, and it was not a good thing since it seemed they might have sent the first letter to a prior address. They were sorry, but too bad.