41 C.F.R. § 301-10.420
When authorized and approved by the agency, employee transportation expenses in the performance of official travel are reimbursable for the usual fare plus a tip which the agency determines to be reasonable for use of a taxi, TNC, innovative mobility technology company, shuttle service, or other courtesy transportation (if charges result). When selecting a TNC, first consideration should be given to the General Services Administration's Ridehail/Rideshare program.