Pressbooks is free to try and use indefinitely, with promotional watermarks in the book files.
When you're ready to remove the promotional watermarks from your book and publish in the ebook and print-on-demand venues, upgrade to either our EBOOK PRO ($19.99) or EBOOK+PDF PRO plan ($99).
This is a one-time charge per book that enables you to revise and export watermark-free files for that book as many times as you'd like, in perpetuity. Upgrading also unlocks the global privacy setting for your webbook and allows you to make your webbook public.
To upgrade, click on Upgrade in the Dashboard and enter your credit card information. (If you have a discount code, be sure to click Verify.)