One price rule for commodities isn't super relevant for consumers though? Commodified markets are well understood so long as there is decent competition.
What we are increasingly seeing on the consumer side of the market - even on grocery items - is price segmentation. Grocery stores (moreso their suppliers) learned that many (most?) consumers are willing to pay much more for staple food items that are not commodities but quite common buys. Like chips or soda or branded packaged foods. They set a regular retail price to 50% more than it was a few years ago over time, and then to capture more of the price sensitive consumers they offer incentives like coupons, in-app deals, random sales, etc. to induce those consumers to purchase.
This is getting to be extremely aggressive and will continue to do so for the foreseeable future. Uber/Instacart for example have plenty of whistle blower insider types who have written about how price segmentation on an individual basis based on personal information and habits happens. Such as the type of credit card on file (Amex holders get charged more), how much gift card credit balance you have, your trends like accepting higher prices once from a given location/destination pair and time, etc.
If you go to the McDonalds drive-thru and simply order at the window you will be likely paying considerably more than the person who has the app installed and orders through that method.
Airline tickets perhaps follow this model as well - browser history and cookies will present a higher price to one consumer vs. another for the same book at exactly the same time. Some court cases are attempting discovery on this recently, so it will be interesting to see if true.
The price of an individual consumer transaction is absolutely set to what the company charging it believes the market will bear. Increasingly that "market" is the size of exactly one consumer.
I listened to a few earnings calls for fast food and consumer staple companies during COVID. Executives were absolutely incredulous that they could continue to increase prices and have it not impact volume of sales much if at all. What was taught in MBA school simply was not reality on the ground, and COVID times exposed this fact. The US consumer at least as a whole has simply lost the ability to price shop and is not as price sensitive as the textbooks say. This may change, but it's the current state.
About the only thing producer prices set is a pricing floor.
