For a brief time in mid-June, it seemed like Costco (NASDAQ: COST) had stumbled badly.The company switched its rewards credit card from longtime provider American Express (NYSE: AXP) to a Visa (NYSE: V) card provided by Citigroup (NYSE: C) on June 20 and the immediate results were a disaster. There was customer confusion, problems at checkout where consumers tried to use their now-inactive cards, and over 1.5 million calls to Citigroup, which had trouble dealing with the call volume.