AITA for reminding another mom that her babysitting wasn't free after she posted about the 1/2 babysitting co-op we are in?

We are in a 10 person babysitting co-op with some moms from my kids' school. We all do a potluck dinner once a month and swap kids for an evening. The babysitting is free, but everyone has a "buy in" of $100 that goes into a group fund to pay for things we need to set up the potluck, like serving utensils, plates, napkins, etc. <br><br>We also have a group set up for carpooling the kids to school in the morning so parents can have a morning off here and there. <br><br>The group chats are mostly set up so parents can communicate who is bringing what to the potluck. I'm one of the main organizers, so I'm always double checking to make sure we have everything, and that if someone is bringing something, they know if it needs to be a serving spoon, or just a regular spoon. <br><br>Sometimes we will comment that a particular serving spoon was shit quality, and we should replace it.<br><br>One of the moms who was a pretty regular attendee of both the potluck, and the carpool posted that she was moving to another part of the city, and would miss us. <br><br>We were all very happy for her.<br><br>Then she posted screenshots of texts she had with a couple of the ladies in the co-op, with them apologizing for not including her in carpool rides. <br><br>She also posted a long post about how she had bought a few things for the group, but had never been reimbursed, and how she had spent $40 on a serving set, and no one had ever said anything to her about it until she posted about it. <br><br>I finally lost it when she posted about how babysitting through the group was supposed to be free, but we had asked for money for various things. I said, "I'm sorry you feel this way, but we did have a buy in, which is bizarre if this was supposed to be free. And we have only ever used money for group expenses."<br><br>She basically went off on me and said that we had used money for stupid shit, and she had only ever used the babysitting. I let it go, since it really wasn't that big of a deal. <br><br>When I got home from work and checked my phone, I had about 20 texts from various people in the co-op that I had upset this mom so much. That some of the other moms were pissed at me for being "stand offish" and not inviting them to things. It went on and on. <br><br>I honestly don't even know where to begin with this. Am I the asshole for pointing out that what she was getting wasn't actually free?