Google has entered the paywall-as-a-service business.

Last November the company announced Contributor, a service that would block banner ads on certain publishers' sites in exchange for people paying a monthly fee.

Google has been emailing details of the service to people who had requested invites to try Contributor. Those details include how much the service will cost and how many ads it's expected to block. Google laid out the monthly pricing tiers included in those emails.

Originally Google had said people would pay $1 to $3 a month to check out participating sites without ads. At that point The Onion, Mashable, WikiHow, Urban Dictionary, ScienceDaily and photo-sharing site Imgur were among the initial publishers on board. Now Google claims that "millions of sites -- everything from small blogs to large news sites" have signed on to not serve ads to Contributor subscribers.

However, as the pricing tiers describe, people who pay Contributor's monthly fee will still see ads on participating publishers' sites. A Google spokeswoman didn't respond to an emailed question asking why not all ads would be blocked.

"We're continuing to test Contributor and recently added some new testers from the waitlist. We're happy with the trial so far but don't have details to share," the Google spokeswoman said in an emailed response.