Something Unpleasant Happened

On Sunday, there was a small event hosted by the store. Since it was free to participate, I was planning to go, but my plantar fasciitis flared up again and the pain got worse. I also heard that I couldn't wear athletic shoes because footwear needed to match my outfit, so I thought I wouldn't be able to attend. I sent a LINE message to decline, and they replied, "A cancellation fee will be charged." When they first invited me, they never mentioned any cancellation fee. They even said it was okay to prioritize other plans if something came up. But then they suddenly said there would be a fee. Hearing that made me feel even less inclined to participate in the future. I can't accept being charged without any prior explanation, and it made me feel really bad. Should I just pay the cancellation fee this time and decide not to participate in anything from now on? I'm starting to lose trust.