Big changes are coming to Costco next week.

Starting Monday, wholesale-store giant Costco isswitching credit card providers to Citi Visa, ending its 16-year relationship with American Express. For those among the Costco’s 81.3 million members who use credit cards for their purchases, it’s a big deal. If you’re one of them, here’s what you need to know.

When does this happen? 

The last day that customers can use Costco’s TrueEarnings American Express will be Sunday. On Monday, those cards will no longer work at Costco or anywhere else. With that in mind, customers need to make sure they aren’t making automatic payments with the AmEx card.

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