But there's an even more basic question about Rubio's Amex use, whether he should have been charging personal expenses to the corporate card in the first place.
When Rubio was using the card, the Republican Party of Florida's policies stated that corporate cards were for "business use only," according to a report by an outside law firm that conducted an independent investigation into credit card expenditures at the state party's request.
So why was Rubio using the card to charge personal expenses in the first place? According to Rubio's camp, nobody knew the policy.
Top Rubio campaign aides point to a February 2010 statement from a party spokeswoman who said at the time "there are no formal party rules or bylaws that govern credit card expenses."
"The (Republican Party of Florida) American Express card is a corporate card and is meant to be used for business expenses, but if personal expenses are charged to the card, the Party trusts that individual cardholders will reimburse the Party for those expenses," the spokeswoman said, according to the 2010 statement Rubio's campaign provided to CNN.
Top Rubio adviser Todd Harris told CNN in an email that the contradictions between what the party spokeswoman said in February 2010 and what the independent investigation found nine months later proves that there was confusion over the rules.
"If there was a policy, no one knew it, not the party spokesperson, not Marco, not former Speaker Dean Cannon who also put personal charges on his card, not anyone else," Harris wrote.
Rubio also said that the Amex bills were "mailed to me at home." But a review of the 18 American Express statements obtained by CNN from 2007 and 2008 showed all but one were mailed to the state party headquarters in Tallahassee, not Rubio's Miami home.
The campaign said that Rubio wasn't talking about statements from those years. Rather, he was referencing statements from 2005 and 2006, Harris said.
Those statements also happen to be the ones that Rubio has not yet made public, though he has been pressed repeatedly to do so, and has promised to release them soon. CNN has requested the documents, but has not received them.
Rubio also explained this week that he went through the Amex bills every month to identify and pay any personal expenses. But in the past, he has explained the personal charges by saying that he sometimes just pulled the wrong card out of his wallet.
So did Rubio purposefully change personal items to his corporate card or were they accidental charges? Harris said it was a mix of both.
"Some were mistakes," he wrote. "Others were made with the intention to pay himself directly to Amex."
In fact, Rubio made the distinction between a credit card and his American Express, which he said was a charge card, implying that it was paid every month. Yet, Rubio went a six-month stretch without making a payment, according to the statements.
Harris said that's because Rubio didn't charge any personal items to the card during that time.