⚠ None of It Is Defensible on Its OwnModerate threat

It is worth being blunt about what the ancillary businesses are not. Gasoline is the most perfectly commoditised product in retail, sold on a price displayed in numerals two feet high to a customer who will cross a road to save three cents. Pharmacy is a regulated dispensing business in which the economics are set by insurers and pharmacy benefit managers. Optical and hearing aids are ordinary specialist retail. Tire installation is a service bay. Costco does all of them competently and none of them uniquely.

Their value is entirely derivative: they exist because they produce visits, and visits produce renewals. That makes the facet a purchased advantage rather than a structural one, and purchased advantages can be outbid. A competitor that decides to sell fuel below cost in a specific market can take the trips back for as long as it is willing to fund the loss — and unlike the assortment discipline or the owned real estate, there is no accumulated barrier stopping it.

Costco's protection is that the ancillary businesses only work inside the membership. Cheap fuel is available to members; the queue is composed of people who have already paid. So a rival matching the fuel price captures a shopper rather than a member, and the trip it buys ends at its own forecourt rather than inside a warehouse.

The exposure that is not hypothetical is regulatory rather than competitive. Costco discloses a False Claims Act investigation by the United States Attorney's Office for the Western District of Washington, opened in January 2023, concerning whether the company presented false claims to the federal government relating to prescription medications¹. Pharmacy was also the largest single positive contributor to gross margin in the third quarter of fiscal 2026², so it is not a small business any more.

Watch warehouse ancillary and other businesses as a share of net sales: $51,170 million of $269,912 million in fiscal 2025³. Rising fast on fuel prices is noise. Rising on volume is the facet working.