Jimi Russell, spokesperson for [NASA](https://www.newsbytesapp.com/news/world/national-aeronautics-and-space-administration)'s Space Operations Mission Directorate, clarified by email that astronauts are on official travel orders as federal employees when in space.
He said they don't get overtime or holiday pay. Their transport, meals and lodging are borne by the agency.
However, they do get daily "incidentals" allowance of $5 per day, equal to per diem payment in lieu of reimbursements.