People want the things in an easy and simply way, they are lazy. For now your target is the everyday consumer who uses single use beverage cups that is also compromised with the environment, so you have to find more value in order to open that target, the key is inside your bonus programs and rewards. You have to give them a huge value that make them not care about taking the mug back to the collection points. In my personal case, I buy the coffee in my way to the university, that is not the same for comming back, so I need a big reward if I have to replan my day in order to taking back the mug, something that means walking more. (I know that you will have many collecting points, but the customers will still having to adapt theirselves) People also love contests, so you can think something related with that. (each coffe that you take gives you more possibilites in the contest) And you can also create a kind of stadistic that shows to the customer how many resources is he saving for the environment, like water, petrol, etc.
I really like the proyect, we need more of this kind of iniciatives. I hope the best for you guys!