The U.S. Secret Service reportedly overpaid a total of $4 million to 2016 presidential campaigns for air travel, reports CNN citing a report by the Government Accountability Office.

The big picture: It's another headache of a financial headline for the Secret Service. Less than a year into President Trump's tenure, the agency hit its overtime cap protecting POTUS and his family.

The details: Eight months before Election Day, the Secret Service realized that "they were using the wrong formula to calculate the costs" of air travel for the campaigns, per CNN.

• Additionally, the agency's policy had been to pay the lower of two options: the lowest available first-class fare or a pro rata share of a campaign charter flight. However, for the latter, it did not require campaigns or charter companies to provide invoices to document the cost of charter flights.

By the numbers: The report shows that an estimated $58 million was spent on air travel related to the campaigns including "two vice presidential candidates and six of the candidates' family members":

• Donald Trump: $7.3 million for 965 flights.
• Hillary Clinton: $7.1 million for 1,317 flights.
• Bernie Sanders: $2 million for 159 flights.
• Ben Carson: $615,567 for 107 flights.