Buyers always think they can get a lower price or commission reduction by contacting the Agent with the yard sign. (The Listing Agent) This idea is not to your benefit and will NOT WORK. Why? The Listing Agent has a fiduciary duty to get the most money for their home seller. They may lose their licenses if they act in bad faith if they talk to the seller about lowering their price for a quick commission. (Regardless of how you have seen business done in the past)
Before the house goes in the MLS, a commission is offered to any Buyer’s Agent who uses the MLS; the commission is already negotiated before you even see the home for sale. (It's "Baked In" already) The seller has already agreed to pay the commission and calculated it into the house sale.
The only way to get a good deal is to use a good Buyer’s Agent (Realtor) with a fiduciary duty to bring you the lowest price that works for you. The Buyers Agent and Seller Agents model creates an “Open Market” scenario that “Shines Sunlight” for buyers and sellers. Note: Real Estate in California should be sold in a “Public Market Scenario” to avoid Serious issues.
The best way to control commissions is to request a Buyers Broker Agreement with your agent. Within this contract, you can control the commission and services of your agent. I prefer to use a Buyer’s Broker Agreement because I want you to know what services I will be performing; this way, you know I am doing a good job. Make sense?