I saw this sign during a nature call on the Parks Highway near Mt. McKinley.
That’s a great sign.
The sign assumes you want to help, or are at least willing to help, and only lack instruction about how. It says that doing this badly has a financial impact, but doesn’t make threats about removing the toilets or replacing them with pay toilets. Then it assumes you’re willing to do something to make the next person’s experience more pleasant, and says how: by closing the toilet lid.
But beyond that, look at the language. It’s not regulatory but invitational. “Please” do this. “Help” with that. Some people would write the sign “Do not dispose of trash in the toilet,” but this is much better.