Charge 50% More or 50% Less

Solution

If someone brings their own reusable cup, they are charged 50% less for their drink. If the store has to provide the cup, they charge 50% more. A normal $4 coffee drink would be $2 for the person with their own cup; $6 if the store also provides a disposable cup.
Additional questions remain:
--1. Should stores still be allowed to offer drinks in inexpensive, disposable cups or should all their drinks be offered in microwavable plastic cups to encourage reuse?
--2. Should customers still receive a discount if they bring in a disposable cup to reuse?
My proposal for both is to have a 6 month introductory period where disposables are still allowed by both store and customer. After that, they are phased out and only microwavable reusable cups can be used.