You seem worried about your shop being "OP," which isn't a bad mindset or anything, but honestly, its issue is the exact opposite. Users with a lot of Poké to spare might buy the basic milk, but the berry-milks are all a net loss. Why would someone pay to have the effect of a berry they already owned be lessened? That's all it is; they give you 50 Poké and a berry, and you give them back a weaker version of the berry. Sure, it also restores energy from the milk, but they're paying for that effect separately and it would have restored that energy regardless. If anything, you should be giving the full effect of the berry in question and a bonus of some sort; otherwise, they're still paying you for nothing.
As an example, an oran/sitrus berry's energy could be upped by 5, and status-curing berries could also grant temporary immunity. Alternatively, since it's an entire jar of milk rather than just one berry, it could have multiple "uses." For instance, you can drink from the jar 3 times until it's empty, and it restores energy and grants the effect of the berry (if applicable) each time. I'd personally recommend something like the latter since it's different from other shops and it's something that would be useful. May wanna reduce the base energy the milk restores to 10 or so if you do that, though; adjusting the prices a bit would be good too, like making Lum and Sitrus milk cost a bit more.
Also, while having 80 stock that resets every day isn't exactly "OP," it's really overkill. You could probably set the stock to 10 and still not have it cleaned out on most days. It's also a bit unreasonable from a purely aesthetic point of view. I mean, can you imagine getting 80 jars of milk from a cow in one day?
Oh, and you could also offer any berries you happen to have on you as a "stock" of sorts that users can pay extra for if they don't have their own. Just make sure to keep it updated.