41 C.F.R. § 302-7.301
Authorization for the shipment of UAB by expedited means
Agencies may authorize the shipment of UAB by expedited means when:
(a) Shipment by a lower cost mode cannot deliver the items being shipped by the time the UAB will be needed by the employee and/or the employee's immediate family;
(b) The employee certifies, and the agency accepts, that expedited shipment of the UAB is necessary to carry out the assigned duties; or
(c) The agency determines that an expedited shipment is necessary to prevent undue hardship to the employee and/or members of their immediate family.