A transition that Costco executives promised would be seamless has turned into a customer service fiasco. On Monday, Costco took the final step in severing its 17-year relationship with American Express and began exclusively accepting Visa for credit card purchases at its stores. [...] Citigroup has logged more than 1.5 million customer service calls about problems with the cards, according to Jennifer Bombardier, a spokeswoman for Citigroup.