Is it permissible for a traveler to combine prayers at the end of the day?

This is a grave reprehensible act, not asserted by any scholar. A traveler is only permitted to combine Zuhr and Asr, or Maghrib and Isha, during the time of either one of them. As for Fajr, it is not combined with other prayers; rather, it is always prayed in its own time, whether one is traveling or at home.