The free app adds the promo signature at the bottom of all outgoing messages.
Not sure why the printer would be using a whole page for it.
BTW, I also have an HP cloud-enabled printer, with its own email, and there are printing apps that support this setup and are much more flexible.
My personal choice is Cloud Print by Paolo Fernandes (yes, I mentioned this before), we worked together to support the printing of HTML messages from my app, with formatting and images.