This refers to having the POSSIBILITY of using templates for your mailings. No to the actual customization of some Mailing template.
So, if you're OK with the default Mailing template that PushRoom generates for you, you don't have to pay for it. If you need one, this will add (full amount, not prorrated) to your monthly fee. Once you create a new Template the FEE will be automatically added and will be there until you stop using this template (by deleting it and all the Newsletters based on it).
Please check the Mailings section of this Help to learn technical information about customizing your Mailing Template.