C172 non flying pilot reported a runway excursion during landing. Flying pilot elected to use reduced flap setting to compensate for bumps and drifted off downwind side of runway after ballooning during landing.

Narrative

On an RNAV approach into ZZZ for runway XX Person A and I experienced a runway excursion. There were winds coming from about 240 @ 6kts but we experienced turbulence upon descending to the IAF. Person A took the flight controls. He was steady on the approach despite the bumps. He put in 20 degrees of flaps but said he did not want to come in too slowly and would land with the 20 degrees only. We touched down smoothly but floated upward a bit. Upon the second touchdown the wheels stayed on the ground but the aircraft sailed toward the right and off the edge of the runway. There was no damage to the aircraft; facilities; or pilots. ATC immediately responded asking if assistance was needed. We first attempted to move the aircraft to the runway but the shoulder was soft and there was not enough traction. ATC asked us to stay on radio while they dispatched someone to come to the aircraft. We shut down and I stayed on the radio while Person A did a walk around check of the aircraft and called Person B.