I just wanted to chime in that I really enjoy this game and agree with the suggestions in this thread. Its a very simple but enjoyable concept for a game and honestly the core gameplay is very solid and just needs a few tweaks as mentioned from this thread. The way the game seems programmed customers won't order any breast milk unless you have a lot stocked up meaning you don't really ever need to worry about it which seems alittle bit of a let down given the main draw of this game is milking the girl. Maybe putting a timer on the customers before they leave and you lost happiness with the girl? Then make more of them require the breast milk so you need to make sure you have a stockpile of it without going overboard and upsetting the barista.
I also agree that purchasing more ingredients makes the game more difficult without adding much benefit. As it stands the main way it seems you fail state is by screwing up orders and letting the barista overfill on milk, which purchasing more ingredients doesn't really help prevent but instead speeds up this process as the drinks get more complex to make. Maybe an overall timer needs to be introduced that if the girl hasn't gotten enough size upgrades at a certain point she leaves? This way you're forced to try and make money fast to get those upgrades?
I think those are the main things gameplay wise that need to be looked at otherwise I think the core gameplay loop is pretty great. The other additions would be bigger sizes for the girl with increased difficulty, other girls to serve as barista with different traits that could affect gameplay, and more outfit options. But I'm sure all of these are already in mind so I just wanna say you guys have done great so far and I'm excited to see what comes later!