Integrated stylus functionality has become one of the big selling points for high-end portable devices, but the big players in this space, Apple, Microsoft, and Google, each charge $99 for their device-specific styluses. That's on top of the many, many hundreds of dollars they already charge for the iPad Pro, Surface, or Pixelbook devices themselves. So this is a risk-free opportunity for would-be stylus slingers to see if they would find such an accessory useful.
The Pixelbook had been $100 off not too long ago and recently went back up to its regular retail price starting at $999, but if you intended to buy the pen anyway, this more or less amounts to the same deal.
At the Google Store, just add the Pen to your cart and the price is deducted at checkout. Amazon and Best Buy are simply offering the Pixelbook and its pen as a bundle.
The Pixelbook Pen is marketed as one of the device's marquee features, though reviews of its usefulness and reliability have been mixed. In his review of the Pixelbook, AP's David Ruddock was not exactly enthusiastic about the pen:
I genuinely don't believe it's worth it unless you know to a certainty that you want to do a lot of stylus work, and that's going to be a personal question. I don't think Google's stylus support on Chrome OS is especially amazing or that the Pixelbook's Pen has particularly great latency or "feel." It's just... fine.
But if you've been considering a Pixelbook because of its stylus integration, but have been put off by the idea of having to drop an extra hundred bucks on it, now's your chance.