That might be true for clothes or food, but even that is dubious, more likely some old wives tale that represented a nugget of truth long ago that has been passed along as a marketing story. Having worked at BB in the past with access to the store cost on many items, I can tell you with 1000% certainty Costco's margins on stuff like electronics and small/major appliances are nowhere close to 15%, even diluting it with comically ridiculous amounts of overhead and cost of business expenses. For starters 95% of name brand stuff is price controlled these days and they literally wouldn't be allowed to sell it 15% over invoice even if they wanted to, the major manufacturers would collude and pull their entire allotment by the end of a business day. Protip, the $3000 77" OLED at Costco that's simultaniously $2000 on Greentoe is still being sold for a handsome profit even at $2000.