How do I go about refusing to work with someone at Costco?

She sounds like a lot of younger employees, & some old ones. If you like your boss, explain what's going on, just don't say you don't want to work with her. Say you're having a problem & you're not sure what to do or say. Then they will think you want to get along & you won't seem like a jerk or troublemaker. It will also make them feel like you need them & think they have good advice & an answer, which maybe they do. If all fails, tape it & show the boss. At least you can prove you were right. Unfortunately, a lot of employees have the same attitude. The old days were MUCH better. People had a sense of responsibility, knew their job & did it well, respected the customers, had pride in a job well done & EARNED their paycheck. They also dressed like an employee, not a customer. Miss all that.

You may come off as a troublemaker,and that is one of the top reasons for being fired ,the top being stealing or not showing up.

ps.

From this question you sound like a trouble maker, not getting along with other employees is a major problem for a company.

Getting rid of those types of people ( that cause workplace disruption) is often in the employers best interest ,because it limits work disruption and bad attitudes that reflect poorly on the brand and in customer service.

I think everyone here except the first person to answer to me is a complete ******* idiot.

Workers don't generally get to pick their assignments. If part of your job is to help out the cashiers when asked, then you can't choose which ones you'll help and which ones you won't.

You can tell your manager. He or she will talk to her. A lot of younger workers are like this. They need to learn to work alone

Quit.

only by giving your 2 weeks notice. Other than that, if a manager tells you to do a job, you do it.

You can't. You can quit, that's it.