Ideas
Having the exact same issue. I'm playing a modified rags to riches and started off with the Home Chef food stand and finally raised enough to open up a modest little cafe that serves food and coffee. Totally shocked when I saw my expensive goods selling out for a one-serving price. I can't imagine this is the intended game play.
Something that could fix it and would be realistic would be to need someone to "tend" the display case - like you would have in a real cafe. Someone that can hand the customer a single serving and charge them. I'm imagining it working the same as the Home Chef Hustle "tend table" action, but without the time limit. I know some people might not like this but for me it would make the game play more immersive and realistic; I kind of hate the idea of people just buying stuff anyway given there's no cash register or anything (we had cash registers in Sims 2!). This would be a nice way to still feel like it's a real business being worked by people, instead of a self-serve shop.
Given that Dine Out was a total bust and we can't really have restaurants due to that (not to mention the intended game play there was disappointing anyway with no cooking, etc.), this feels extremely bad. This was my chance to "pretend" to have something restaurant adjacent and now I have to scrap the entire thing until this is changed.