Dear @Rajsourav
To raise the issue of extra charge you were levied, I recommend you lodge a complaint with the Bihar State Road Transport Corporation (BSRTC) by calling the respective bus depot where you boarded the bus or the control room of the roadways.
You can also call at the BSRTC helpline number 9798047742 / 9508633131 , or email bsrtcreports@gmail.com.
They will be able to investigate the issue to take appropriate action and might even be able to refund the extra charge you paid.
Still, have doubts? Let me know if you need more help regarding BSRTC services.