a couple years ago, I called the reservation line 2 hours before my reservation, and was able to get it cancelled without penalty as a courtesy. The park was crazy busy, hot, we were all exhausted, and we went back to our hotel room to rest, and didn’t feel like going back to the park.
what I feel is that if you make the effort to let them know, there is a decent chance they will waive the fee as a courtesy. Don’t count on it though.