The price is checked server side. You have to pass the price somehow to the client in order to display it. This is no different than any other ecommerce site, except that you can see it in a GET param.

I can "change the price" of anything on Amazon with dev tools, but that won't help me when I go to buy it. I appreciate the concern because amazingly this has been a real vulnerability on sites before (recently on a crypto exchange I think).

Storing it, or passing it by the user can be suitable if the data is signed