You do not need to disclaim your commission.
Nor need you explain that commissions are negotiable or that they are not set by whomever or that there is no "standard" commission.
I know everyone is worried about antitrust violations. But stating YOUR OWN commission, both on a website and in a listing presentation, is not by a long-shot an antitrust risk or violation.
You are not, by making a presentation or advertising your price (which is what a website does), engaging in restraint of trade or colluding with anyone to set a price. You are just stating what YOU charge. You're quoting a price, and you are free to do so. You are not saying, you are not implying, that this is what all real estate commissions are. You are not saying that other brokerages or agents should or must charge what you charge; you are not asking any to do so. You are entirely free to advertise your price, and all it has to be is truthful--not disclaimed about negotiability or how there's no standard commission.
And, by the way, *my* commission is *not* negotiable. Commissions are "negotiable" if one uses a very loose connotation of "negotiable," that is, in that every transaction that includes a commission (that is, every listing contract) states what the commission will be and has to be agreed to by both parties to the contract. But that means that they are not mandatory or required by law to be at a certain level or amount. It doesn't mean YOUR commission is negotiable.
If you don't want to negotiate on commission---and guys, most of us do not negotiate it most of the time, we tell the client what we charge and they accept it or they reject it (THEN we may either negotiate or decide not to enter into the listing agreement). You are free to make your offer of commission, at whatever level you practice, a take-it-or-leave-it offer. And to advertise and present it as such.