Please Help: service tax

the facts are as under:
client is a middle man in between cab operator and service receiver (consumer), providing online portal for booking;
consumer are of two type, 1 - Corporate and 2 - other (individual/family);
my client receives advance / booking amount sometime more/less than its commission through online booking;
rest amount is received by the driver in case of other consumers because in case of corporate consumer monthly bill generated including commission and operator charges and service tax calculated accordingly on whole amount receivable @ 4.83% after abatement;
monthly reconciliation is done with cab operators for our commission or their charges on pre-settlement basis;
Issue:
we generate e-ticket for advance booking/payment i.e. almost our commission but rest amount is received by the driver on completion or during the trip. We dont give invoice for full amount as consumer dont call for it. So, in this case, should we pay service tax on the advance amount only or on the full amount (including part payment received by the driver of cab operator)?