Holy crap those are some insane dues! I had a similar experience working for a grocery store at that age. I am pro-union since I have seen the good it does for trades, but it seemed scummy trying to convince me to join the union as a kid who was on his parents insurance at the time and didn't need benefits.
Then why try to convince me to join if I'm paying into insurance and strike funds that I will never see any benefit from? I understand collective bargaining but it seems like they just wanted to trick high school kids into giving away 15% of their paychecks. Also even though the store had high union membership, my managers still pulled crap like calling me to the register during my legally mandated 15 minute break.
Funny enough, the person who replied to you was wrong. I didn't get health insurance. They were dues. The benefits I mentioned were guaranteed raises that I knew I wouldn't get because I didn't plan to work there for more than a summer.