Officials say a computer hardware problem caused more than 800,000 credit card and debit transactions to be double-billed or even triple-billed last week.
A spokeswoman for the electronic payments processor First Data Corporation says the excess charges have been reversed, but the reversals may not show up in some accounts until Tuesday.
Anyone who made a charge with either of the two cards on Wednesday "should check their statements."
First Data is making phone calls to all of the affected customers.
But if you have any questions, you can call them at 1-888-893-0626.