If by this you mean that standard supply-demand economics can't model price discrimination, which is what's going on here, that's not correct. See, for example, Chapter 10 of David Friedman's Price Theory, where he models price discrimination using supply and demand curves just fine. In terms of this kind of analysis, price discrimination is a way for sellers to try to transfer as much as possible of what would otherwise be consumer surplus, to themselves.
And the buyer tries to pay as little as possible. Negotiating is a skill well worth learning (lots of books on it).
This discussion is perverse. Negotiations require leverage, which the average grocery buyer in the USA does not have. As a society we don't benefit from min-maxing absolutely every opportunity.
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Government stores are fine then?