Am I the bad apple for charging my mom $25 to pick her up in her car?

So I am a female in my early 40s. My mom just turned 70 last year, and due to trauma she had when I was a child (two car accidents a few years apart), she can’t drive on the freeway without receiving a migraine. She and I both care for my autistic brother who has high support needs. For the most part, he can take care of himself, but he needs constant supervision. So every other week I spend three days at her house. While she takes a respite at a friend’s house. I will admit I love my brother, but with his need for constant supervision, a lot of his care is emotional labor. The deal is her friend picks her up (usually on a Wednesday). She spends three days there, and then I take my brother to a movie that Friday and since her friend’s house is on my way home, I pick her up. Now we agreed a while back that if I have to make a special trip, that would be $25. I myself do not have a car. I use her cars when I am staying at the house caring for my brother. When we first made this deal, she told me that wasn’t right since I’m her daughter and they’re her cars, she should not have to pay me anything, but it is my time. Next time I go there, I will be unable to be in the neighborhood of her friend’s house, so I will be making a special trip, which would be the $25. Today she calls me and tells me it should be less, somewhere between $ 12.50 and $ 17, because it is her car. Besides picking her up, I also am basically her accountant. My dad passed 2 1/2 years ago, and for reasons I won’t get into here, managing her finances fell to me. I manage her mortgage payments, her property taxes, her car payments, her credit card payments, and utilities. I get paid for this as well. So with everything I manage for her financially, am I being fair sticking to the $25 agreement or am I being the bad apple?