I say do something nice for yourself, you earned that money. Yes, my dear, you went out of your way to ensure that someone else didn’t have to go through a hassle. You did it without any intent of monetary gain. What you did was worth the $20 from the gentleman’s perspective (I’d have done the same).
So, spend it on something that makes YOU feel good. If that’s charity, so be it. But don’t feel bad if you use it for a nice meal or some well deserved pampering.