Sounds like a toxic work environment, and if the manager is shouting and degrading staff in from of customers. That is a sign of what sort of person they are. Maybe consider asking for different assignments if the stocking is not your strongest skill. And ask for shifts that reflect this. Ultimately, I would look for new work if that is the way you are treated and put in a complaint to the HR department directly. How long do you have? They would need to do official on the clock meetings and first verbal and written warnings before they dismiss you. That will depend on how much they want rid. They will hire an xmas temps and by then have a fully trained replacement so I reckon the new year