Service tax on aggregator of transportation if goods

We are engaged in aggregation of transporters & suppliers. We are providing transportation of goods of our customers from their location to destination (buyer / godown etc). We do not own trucks. We engage transporters for this service and they issue consignment note (Bilty / GR) and our name is not mentioned on the GR. We issue our invoice to supplier for payment, he pay us and after keeping our commission we pay the amount to transporters. We are a private limited company. Amount on our invoice is same as mentioned on GR / Bilty of transporters. We don't disclose our charges to suppliers. The amount to be charged from our customers (suppliers) is mentioned as transportation charge in our invoice. Our invoice contains all info of GR I.e. Consignee, consignor, truck no, loading place and destination of goods, transporters name, GR no, goods details (value, no of packs, weight, descripttion etc). Are we liable to pay service tax whole invoice amount or only our commission amount?