We use to charge a supply charge. We had some customers complain. Plus, New York has a few restrictions on how to charge supply charges and enviornmental charges. According to New York, you can charge supply and other charges only if you document what the charges are for, and, you are only allowed to recoup your expenses, not generate a profit.
It may be different where you are.
It looks like the supply charge really works for you, that is great!