Is there a fee for using Pages commercially?

There is no fee for using pages. It was pre-installed on your Mac with a host of other software for your use, or for participating in a team such as you would find in a business.

In this Apple's publicity page for the application they promote the use of Pages in collaborative teams:

Work together in the same document, from across town or across the world. You can see your team’s edits as they make them — and they can watch as you make yours too. You can even get notifications when people join, edit or comment. Just click or tap the Share button and invite people to join

and there is also a page outlining the collaborative abilities of Pages and the other applications in the iWork suite: Collaborate on documents in Pages, Numbers, and Keynote - Apple Support

No fee. The official Apple license for Pages on macOS is here (direct PDF link). There do not appear to be any commercial restrictions on it as long as one uses it in the manner prescribed in that agreement.

Although Apple did pre-install Pages, Numbers, and Keynote in late 2013 into 2014, they have not done so since, in my own new or refurbished purchasing experience with products directly from Apple. Apple resellers may choose to bundle these Mac App Store-sourced applications as a sales incentive.

Thanks for that correction VikingOSX. I'd not noticed Pages/iWork wasn't loaded and when I got my new iMac last year I blissfully set it up and didn't notice even then.