The thing competitors can't easily copy that keeps your customers from leaving, whether that's scale, brand, network effects, switching costs, or proprietary data.
Costco's $60-$120 annual membership fee looks like a small revenue line, but it's the entire moat. Members renew at over 90% year after year, which lets Costco price products at near cost and still print profit on dues. Competitors can copy the warehouse format, but they cannot copy 70 million households who already paid to shop there.
