The banner at the top of Costco’s Facebook page reads “Costco + Visa = ♥,” but for many customers, the first few days of the retailer’s new charge-card alliance have been anything but heartwarming.

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. For more than 11 million customers, the switch meant cutting up their Costco-branded American Express cards and swapping them for a new Visa card issued by Citigroup.

A transition that executives promised would be seamless has turned into a customer service fiasco.

Citigroup has logged more than 1.5 million customer service calls, according to Jennifer Bombardier, a spokeswoman for Citigroup.

Costco’s social media pages are filled with thousands of scathing complaints. Many customers say their new cards have not yet arrived, and some report problems activating or using their cards or getting access to their accounts online.