Is there a to-and fro trip Xian - Qianling taxi service at a fixed fare?

There is no fixed rate for that route. In casing of get decieved, you are suggested to turn to a reliable car rental company with good reputation.

On the website you are visiting, I know they have car rental services, which can take you fro and to Qian Mausoleum. Try to contact with them, which may help to smooth your trip.