Google is displaying a new button/feature on Gmail called Paper. Yeah, I almost fell for it too.
From the FAQs:
Is it free?
Yes. The cost of postage is offset with the help of relevant, targeted, unobtrusive advertisements, which will appear on the back of your Gmail Paper prints in red, bold, 36 pt Helvetica. No pop-ups, no flashy animations—these are physically impossible in the paper medium.
LOL, Google. LOL indeed.
Update: They've doubled down on the foolishness and started TiSP. Read it here for yourself.