GST registration for travel business

We do not sell travel package and are only taking bookings for rail / air / bus tickets. We charge a nominal amount as extra for getting these bookings done. This extra amount is not shown separately in our invoice and we charge a lump sum from customers. Our turnover is below 40 lacs (considering only commission part).

A) Do we need to get registered for GST?

B) What methodology should we adopt to calculate accurate revenue as charges from customers is not a fixed amount / %age?