brainsX

So your first priority is to treat everyone fairly.  Ask yourself ”am I disadvantaging one client in order to advantage another?”.  As long as you are not disadvantaging any of your clients, you can offer ‘premium’ services to clients that are willing to pay extra as long as:
a) you fully disclose this policy to all clients, and
b) the premium service is made available to all clients - i.e. the client chooses if he/she wants premium service (you’re not making that decision for them by selectively offering premium service to only some of your clients).
So if you are in some way neglecting client A to offer premium service to client B, you are violating the standard.
If you develop a special monthly report on topic X which you send only to prefered clients (but you fully disclose to all clients that this report is a premium service which all clients can receive if they upgrade to prefered status), you are not violating this standard.