We were having such a good time watching people sign up today, we figured we should just keep the fun going and extend the sale until Sunday.
Also, there was some confusion about upgrading your existing free account to a paid account. We had originally written the code to apply the discount only for new signups, but that was totally unfair, so the sale now works with free accounts as well. If you go to your account page, you can switch to a paid plan without entering a credit card.
Sign up the entire family!
Update: If you already upgraded before we rolled out the updated code and received a receipt, just ignore it.