Big-box stores can feel like money traps, bright signs, huge carts, and “deals” everywhere you look. But if you know where to look, some of the best things they offer don’t cost anything at all.

You walk into a big-box store for one thing and walk out $187 poorer. It happens fast. Snacks, school supplies, a “quick” home project, and suddenly your cart is a financial crime scene. What most ...