I see to many people here that are making minimum wage and dealing with shitty managers and customers. Stand up to your boss and the shitty customers. The worst they could do is fire you and you have to cross the street to find another min wage job.
You have managers for a reason. If you are a cashier and a Karen starts demanding you accept her 5 year old coupon, just grab the manager and let them deal with her. Don’t over explain things to pissed off customers. “I’m sorry that’s policy, if you would like, you can step a side and a manager will be with you shortly”. You don’t get paid enough to deal with BS.
Don’t take on any extra responsibility. If your dog dies your employer will tell you to get over it and be at the office tomorrow at 9am. So when your employer is telling you that you have to come in on your day off because of staffing issues tell him “oh that sucks for you, I’m not available, good luck”.
And lastly, don’t forget that no good deed goes unpunished. If you help your manager once to (example) clean the windows, guess what, now you are the designated window cleaner because you are good at it.