What is a decent tip for "carhops" that bring a burger and Coke to your car?

This problem sort of bothers me. For decades I’d give them a quarter. Then after I came into some cash I started giving them fifty cents. I rationalized, “if everyone gave them fifty cents they’d have a nice day.” Right? Or is that too cheap also?
By the way: this “problem” is super way down on my list of concerns!