To join right away, go to http://wiz1.us/fpsbeta.
We've been working on upgrading the underlying stack powering FillPDF Service behind the scenes, and we want to get some early feedback.
Up until now, FillPDF Service has primarily been a companion service to the FillPDF Drupal module. The next version looks to change that.
The biggest new feature will be a REST API (it's currently XML-RPC, which is very outdated). The existing features, such as flexible overage billing, will remain (although billing-related features will not be available right away). Participants will have a fixed usage limit, which we will be able to raise for you manually (paid by invoice and payment card) if necessary.
You can join the private beta at http://wiz1.us/fpsbeta. We will invite you when we're ready to have people start testing.