With Trump’s tariffs deemed illegal by the Supreme Court, Costco stands to win big both financially and on reputation

Costco Wholesale has won the loyalty and almost religious devotion of tens of millions of members thanks to one tenet in how it has run its business since its founding decades ago. Everything it does is aimed at keeping costs low so it can offer shoppers the deals on top merchandise that keep them coming back in droves week after week.

It was in that spirit that Costco last autumn became one of the first major companies (along with the likes of Revlon and motorcycle-maker Kawasaki) to...