Darn, I hate these discussion. Everyone's playing games with words. With semantics.
Is there a fixed rate? No. That's illegal. That's a violation of the antitrust laws. Brokers or agents can't get together and agree on a specific rate.
Is there a going rate? Sure. I'll bet 80% of all the agents in Carmel will quote you the same rate. And most of the variation will be between full-service brokerages and discount brokerages. That rate might be slightly different (typically by one percentage point) in different cities or states. So if it's x% in Carmel, it might be x+1% in San Francisco or it might be x-1% in Dothan, Alabama. (I'm just making that up; Dothan might be higher than average and San Francisco might be lower. The point is that you'll see some slight fluctuation in different parts of the country.)
Are commissions negotiable? Yes...and no. Since they can't be set (that'd be illegal), you can always ask for less. That's your absolute right. But it's also the right of the agent to say "no." And in many brokerages, agents know that their broker isn't going to offer a discount just because someone asks.
Now, there are some strategies to negotiating commissions. For instance, if you're selling one house and buying another, that gives you extra leverage and the prospect of the agent (who'd handle both the sale and purchase) earning two commissions. You have a better chance of succeeding at negotiating in a case like that.
Back to your question: It's likely that most brokerages in Carmel charge the same commission. But I could be wrong. You can determine that by calling 3 or 4 agents with different brokerages in Carmel and asking what commission they charge. See if you come up with the same number. If so, that answers your question. That'll require maybe an hour's worth of work on your time. But...considering the commission you'll pay--could be $6,000, $12,000, $18,000, or more--it might be worth it.
Except....final comment here...don't base your decision on which Realtor to use on the basis of commission. Base it on knowledge, expertise, and personal chemistry.
Hope that helps.