The Cashier was an unknown woman who was heading a supply store in Yellowstone National Park. She was played by Eve Harlow.
Biography
She was seen when Jackson Curtis asked for bug spray for his children, then in the store, the TV was on showing the breaking news of a small earthquake/fault hitting a superstore in Los Angeles.
She was presumably killed during the Yellowstone Caldera's eruption either after the Yellowstone Caldera spurred lava bombs all over the area or when the ash cloud was 'swallowing' Yellowstone.