I currently work part-time at school creating content/curriculum for a difficult student. We've made huge leaps in certain areas. What I've found incredibly useful (after loads of approaches) was adapting the student's interests with STEM subjects. Following that, I get the student to adopt a mentor role, and teach what he has learnt to another student or back to me. This two prong process has been helpful to cement concepts and promote responsibility.
If you have time check out the concept, "Absorptive Capacity". Its used in management literature to explain a firm's ability to identify external knowledge , assimilate that knowledge, integrate it, and apply it for commercial benefit.
In an education context, I have found it useful for mapping domain knowledge of the student then leveraging that knowledge to give me the bones to teaching new knowledge.
I really like your tight target audience and focused solution.
An experience (my user journey) and a question about hygiene.
Experience: In my experience, most of the time, a staff person at a cafe wilI ask if I want sugar in my takeaway coffee. I assume to save me taking the lid off myself. The exception is when I purchase a coffee for someone who isn't there that I grab the extras. It is only sometimes I have to put the sugar in myself - e.g., if the till person forgets to ask. Perhaps the cafes I go to are different, but thats the typical flow of things in my country, NZ.
In contrast, when I purchase a coffee to have at a cafe the decision whether I have a sugar defaults to me - sugar sachets, cubes etc are on the tables. Sometimes, I take a couple of sachets out of habit. However, they end up at the bottom of a bag or under a car seat.
Another thought: Presumably, people who purchase a hot beverage from a cafe will add sugar within a certain period while their beverage is hot. At this point, the person who purchases the hot drink has a predicament (given they have grabbed extras like stirrer and sugar sachets): a) they are still in the cafe and dispose of the sachet(s) easily, b) they leave the cafe, then add sugar and when nobody is looking they litter, c) they hold onto to the rubbish until they find the nearest bin, or d) don't have any extras to dispose of - your idea.
Question about hygiene: Do people mind touching something that is going to dissolve into their hot beverage? And if so, is there a way to stop that from happening if you do have an issue with it?