Let me see if I understand this correctly: you are making print copies of websites without their permission?

Even if you did not take a percentage of the cut, or charge any money, this would be a per se copyright violation. (Instapaper itself rests on shaky fair use grounds, close to but not quite over the edge into copyright infringement). You've jumped into the chasm.

If your service was intended for websites to allow people to make customized print copies, it would be okay, because the copyright owner would be opting into your service. The website would be, explicitly or in effect, providing a copyright license allowing for the service/users to make print copies of their website.

And that's assuming you are in the U.S. If you're in Europe, fair use laws are far less permissive.