I would talk to your scheduling ETL. I was scheduled an extra shift in flexible fulfillment that put me 4 hours over my max and when I mentioned it to my ETL (when she asked when I was working this week), she was pissed. She sets the schedule based on people's availability (maxed out these days) and keeps having issues with her people being scheduled when they shouldn't be by other teams.
As others have said, backup cashiering is part of the deal, but they shouldn't be scheduling you as a cashier on a whim, so check with your ETL and see if they are aware. At least that's not cool at my store.
Cashiering stresses me out, too. We're a SuperTarget and I'm someone who personally excels at grocery bag tetris when bagging my own groceries but guests have different preferences and never put stuff on the belt in the right order and I always feel like I'm bagging stuff wrong and that I'm missing things (cartwheel and coupons and REDCards, oh my!).
I used to enjoy cashiering, back in the day, but there's so much MORE these days to keep track of and when I'm new and not cashiering often, on a borrowed register that has something broken or missing half the time, only up front when it's busy and everyone is stressed....gah! Do not want! A full shift cashiering would give me an ulcer. Our guests are generally lovely, though, thank all the gods.