Headphones on, music up, the loser completes a full grocery run while dancing continuously through every aisle, produce to checkout. Strangers hear nothing, see everything. The receipt proves the full shop; the footage proves the dancing.
Budget note: this one costs nothing but dignity, which makes it a strong pick for leagues that would rather spend the pot on the champion than on the loser's downfall.
Seen in the wild: fantasypunishments.com