(This is what happens when my friends make up completely random AU's. Now, I present to you Shoptale)
Frisk ran into Walmart. They wanted to get away from their parents for now. After running inside, the automatic doors slid shut loudly behind them. Frisk looked around. They were in the garden area. That's not normally the entrance to the store, but whatever. A houseplant greeted them. It was talking about sharing money since their wallet was small and empty.
Instead of cash, Frisk was trapped in a shopping cart. It laughed dementedly, saying, "In this store, it's BUY or BE BOUGHT." They cried for help a strange goat lady threw a box of hot spices at the houseplant. It flew over the rows of items, giving a weird squeak.
"Greetings, my child. I am Toriel, caretaker of the outdoor section. Now, come with me. I will guide you through the puzzles." She took Frisk to the next area. "This is a clearance rack. Please familiarize yourself with the sight. You'll need to save your money in the future." Frisk nodded. Toriel left, saying she had to go find a few items before she'd take Frisk to her 'home.' Frisk was given a cellphone (much to their delight) and immediately left once Toriel disappeared.
"How do you get out of here?" Frisk muttered as they wandered. This place was huge considering it was just the gardening area. Several monsters tried to attack them, but with actions and random gifts (that they didn't even buy) they left the child alone. And Frisk got money from the happy monsters after fighting them, so maybe at one point they'd get to buy something.
But when they reached the other end of the gardening area, Toriel was there, with an outdoor grill trying to make a pie. She was surprised to see them, and they went in a tent of their own, went in a sleeping bag, and slept soundly. (Even though they were only there for like an hour. Frisk, are you really that tired out from shopping?)