Expenses for which payment through the Government contractor-issued travel charge card is impractical (e.g., vendor does not accept credit cards) or imposes unreasonable burdens or costs (e.g., fees are charged for using the card) are exempt from use of the travel charge card. Your agency may also exempt an official travel expense when it is necessary in the interest of the agency (see § 301-51.4).
41 C.F.R. §301-51.2