Mini rant, do you deal with frustrating stuff like this?

So there is an event for my SS's school. Apparently bio parents and siblings are free. But when it came to me, I was told I need to buy an extra ticket for myself, as I am an "extra" guest. We aren't even including any siblings either, because the other kids can't come along that day.

As far as I know, I'm the only step-parent in the bunch. That's a really shitty thing to do to someone. It says a lot what they think about someone like me. They have no problem accepting donations and gifts that I may give, but they gotta get that $15 from the freeloading outsider for the get-together.

It's a very frustrating and demoralizing feeling.