The service now works at more than 200,000 stores

Owners of the iPhone 6 registered more than 1 million credit cards on Apple Pay in the first three days the tech giant offered the service, Apple CEO Tim Cook said Monday.

Cook’s claims of the mobile payment system’s rapid pickup came after it emerged large retailers including Rite Aid and CVS pharmacies would not allow the payment system in their stores. Those companies offer their own mobile payment system. Cook described that decision as a “skirmish” and said he remained confident in Apple Pay.

“You are only relevant as a retailer or merchant if your customers love you,” he said, during an interview at a Wall Street Journal conference. “It’s the first and only mobile payment system that’s easy, private and secure.”

Apple Pay, which launched Oct. 20, works with the six largest credit card companies and can be used at more than 200,000 storefronts in the United States.