Your client is liable to pay service tax on 25% of the total gross amount under reverse charge mechanism, if ALL the following conditions are satisfied :
1. If gross amount charged on a consignment transported in a single goods carriage is or exceeds one thousand five hundred rupees
2. If the the goods transport agency in the consignment note issued makes a declaration that neither credit on inputs or capital goods [now input services also] used for provision of service has been taken nor the benefit of Notification no. 12/2003-Service Tax has been taken by them.
Further, exemption from payment of service tax on GTA service is provided only in the following cases:
1. Vegetable
2. Fruit
3. Eggs
4. Milk
5. Food Grains and Pulses.